> Are each of these tests testing a different code path

The first test is exactly problem, mentioned in the issue: correct O-term, but wrong series coefficients.
The second test, sqrt(sin(x**3)).series(x, 0, n) with n=19 and 20, works in master now (I mean, it doesn't raise an exception).  But for n=20 - the last series term is wrong too.  I think, the reason is same - wrong nuse calculation.