Buggy python submission (verdict: Time Limit Exceeded):
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [[0 for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    # 長さを求める
    for i in range(len(s)):
        for j in range(len(t)):
            if s[i] == t[j]:
                dp[i + 1][j + 1] = max(dp[i][j] + 1, dp[i + 1][j], dp[i][j + 1])
            else:
                dp[i + 1][j + 1] = max(dp[i + 1][j], dp[i][j + 1])

    ans = ''
    i += 1
    j += 1

    # 復元する
    while i >= 0 and j >= 0:
        if dp[i][j] == dp[i - 1][j - 1]:
            i -= 1
            j -= 1
        elif dp[i][j] == dp[i - 1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j - 1]:
            j -= 1
        else:
            ans = s[i - 1] + ans
            i -= 1
            j -= 1

    print(ans)

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()

Fixed submission:
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [[0 for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    # 長さを求める
    for i in range(len(s)):
        for j in range(len(t)):
            if s[i] == t[j]:
                dp[i + 1][j + 1] = max(dp[i][j] + 1, dp[i + 1][j], dp[i][j + 1])
            else:
                dp[i + 1][j + 1] = max(dp[i + 1][j], dp[i][j + 1])

    ans = ''
    i += 1
    j += 1

    # 復元する
    while i >= 0 and j >= 0:
        if dp[i][j] == dp[i - 1][j - 1]:
            i -= 1
            j -= 1
        elif dp[i][j] == dp[i - 1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j - 1]:
            j -= 1
        else:
            ans = s[i - 1] + ans
            i -= 1
            j -= 1

    print(ans)

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [['' for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    for i in range(len(s)):
        for j in range(len(t)):
            a = len(dp[i][j]) + 1
            b = len(dp[i + 1][j])
            c = len(dp[i][j + 1])

            if s[i] == t[j]:
                max_abc = max(a, b, c)
                if a == max_abc:
                    dp[i + 1][j + 1] = dp[i][j] + s[i]
                elif b == max_abc:
                    dp[i + 1][j + 1] = dp[i + 1][j]
                elif c == max_abc:
                    dp[i + 1][j + 1] = dp[i][j + 1]
            else:
                max_bc = max(b, c)
                if b == max_bc:
                    dp[i + 1][j + 1] = dp[i + 1][j]
                else:
                    dp[i + 1][j + 1] = dp[i][j + 1]

    print(dp[-1][-1])

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()

Fixed submission:
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [[0 for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    # 長さを求める
    for i in range(len(s)):
        for j in range(len(t)):
            if s[i] == t[j]:
                dp[i + 1][j + 1] = max(dp[i][j] + 1, dp[i + 1][j], dp[i][j + 1])
            else:
                dp[i + 1][j + 1] = max(dp[i + 1][j], dp[i][j + 1])

    ans = ''
    i += 1
    j += 1

    # 復元する
    while i >= 0 and j >= 0:
        if dp[i][j] == dp[i - 1][j - 1]:
            i -= 1
            j -= 1
        elif dp[i][j] == dp[i - 1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j - 1]:
            j -= 1
        else:
            ans = s[i - 1] + ans
            i -= 1
            j -= 1

    print(ans)

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [['' for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    for i in range(len(s)):
        for j in range(len(t)):
            a = len(dp[i][j]) + 1
            b = len(dp[i + 1][j])
            c = len(dp[i][j + 1])

            if s[i] == t[j]:
                max_abc = max(a, b, c)
                if a == max_abc:
                    dp[i + 1][j + 1] = dp[i][j] + s[i]
                elif b == max_abc:
                    dp[i + 1][j + 1] = dp[i + 1][j]
                elif c == max_abc:
                    dp[i + 1][j + 1] = dp[i][j + 1]
            else:
                max_bc = max(b, c)
                if b == max_bc:
                    dp[i + 1][j + 1] = dp[i + 1][j]
                else:
                    dp[i + 1][j + 1] = dp[i][j + 1]

    print(dp[-1][-1])

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()

Fixed submission:
import sys
from io import StringIO
import unittest

def resolve():
    s = list(input())
    t = list(input())

    dp = [[0 for _ in range(len(t) + 1)] for __ in range(len(s) + 1)]

    # 長さを求める
    for i in range(len(s)):
        for j in range(len(t)):
            if s[i] == t[j]:
                dp[i + 1][j + 1] = max(dp[i][j] + 1, dp[i + 1][j], dp[i][j + 1])
            else:
                dp[i + 1][j + 1] = max(dp[i + 1][j], dp[i][j + 1])

    ans = ''
    i += 1
    j += 1

    # 復元する
    while i >= 0 and j >= 0:
        if dp[i][j] == dp[i - 1][j - 1]:
            i -= 1
            j -= 1
        elif dp[i][j] == dp[i - 1][j]:
            i -= 1
        elif dp[i][j] == dp[i][j - 1]:
            j -= 1
        else:
            ans = s[i - 1] + ans
            i -= 1
            j -= 1

    print(ans)

class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)
    def test_入力例_1(self):
        input = """axyb
abyxb"""
        output = """axb"""
        self.assertIO(input, output)
    def test_入力例_2(self):
        input = """aa
xayaz"""
        output = """aa"""
        self.assertIO(input, output)
    def test_入力例_3(self):
        input = """a
z"""
        output = """"""
        self.assertIO(input, output)
    def test_入力例_4(self):
        input = """abracadabra
avadakedavra"""
        output = """aaadara"""
        self.assertIO(input, output)

if __name__ == "__main__":
    # unittest.main()
    resolve()