Issue 1:
This doesn't happen every time.

Using tip (bbc4351).

# crypto/x509
panic: runtime error: slice bounds out of range
fatal error: panic on system stack

runtime stack:
runtime.throw(0x854ad0, 0x15)
/home/ksh/work/os-code/go/src/runtime/panic.go:527 +0x96
runtime.gopanic(0x7ce940, 0xc20800e150)
/home/ksh/work/os-code/go/src/runtime/panic.go:354 +0xb9
runtime.panicslice()
/home/ksh/work/os-code/go/src/runtime/panic.go:18 +0x49
runtime.gcInstallStackBarrier(0xc208000160, 0xc2084b1c78)
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:510 +0xae
runtime.scanstack.func1(0xc2084b1c78, 0x0, 0x1)
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:365 +0x9f
runtime.gentraceback(0x75d275, 0xc2080911b0, 0x0, 0xc208000160, 0x0, 0x0, 0x7fffffff, 0xc2084b1d90, 0x0, 0x0, ...)
/home/ksh/work/os-code/go/src/runtime/traceback.go:336 +0xa7e
runtime.scanstack(0xc208000160)
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:380 +0x34e
runtime.gcphasework(0xc208000160)
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:273 +0x6d
runtime.stopg(0xc208000160, 0x3)
/home/ksh/work/os-code/go/src/runtime/proc1.go:395 +0x8e
runtime.markroot(0xc208020000, 0x5)
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:138 +0x1ef
runtime.parfordo(0xc208020000)
/home/ksh/work/os-code/go/src/runtime/parfor.go:95 +0x12a
runtime.gcscan_m()
/home/ksh/work/os-code/go/src/runtime/mgcmark.go:37 +0xde
runtime.gc.func1()
/home/ksh/work/os-code/go/src/runtime/mgc.go:843 +0x5c
runtime.systemstack(0xc208026000)
/home/ksh/work/os-code/go/src/runtime/asm_amd64.s:262 +0x7c
runtime.mstart()
/home/ksh/work/os-code/go/src/runtime/proc1.go:656

goroutine 5 [garbage collection scan]:
runtime.systemstack_switch()
/home/ksh/work/os-code/go/src/runtime/asm_amd64.s:216 fp=0xc20802fce8 sp=0xc20802fce0
runtime.gc(0x0)
/home/ksh/work/os-code/go/src/runtime/mgc.go:855 +0x1d9 fp=0xc20802ff90 sp=0xc20802fce8
runtime.backgroundgc()
/home/ksh/work/os-code/go/src/runtime/mgc.go:775 +0x3d fp=0xc20802ffc0 sp=0xc20802ff90
runtime.goexit()
/home/ksh/work/os-code/go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc20802ffc8 sp=0xc20802ffc0
created by runtime.startGC
/home/ksh/work/os-code/go/src/runtime/mgc.go:748 +0x147

goroutine 1 [syscall]:
syscall.Syscall(0x4, 0xc20849bef0, 0xc2085886c0, 0x0, 0xffffffffffffffff, 0x0, 0x2)
/home/ksh/work/os-code/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.Stat(0xc20849be30, 0x25, 0xc2085886c0, 0x0, 0x0)
/home/ksh/work/os-code/go/src/syscall/zsyscall_linux_amd64.go:1529 +0x8b
os.Stat(0xc20849be30, 0x25, 0x0, 0x0, 0x0, 0x0)
/home/ksh/work/os-code/go/src/os/file_unix.go:154 +0x6a
cmd/internal/obj.Access(0xc20849be30, 0x25, 0x0, 0x2)
/home/ksh/work/os-code/go/src/cmd/internal/obj/util.go:173 +0x8e
cmd/compile/internal/gc.findpkg(0xc20845a3f0, 0xc, 0xc20849be30, 0x25, 0xc9a300)
/home/ksh/work/os-code/go/src/cmd/compile/internal/gc/lex.go:605 +0x8a9
cmd/compile/internal/gc.importfile(0xc208092a50, 0x26a)
/home/ksh/work/os-code/go/src/cmd/compile/internal/gc/lex.go:711 +0xc2d
cmd/compile/internal/gc.(*yyParserImpl).Parse(0xc208032038, 0x7fbad15be728, 0xdc0e10, 0x0)
go.y:292 +0x2008
cmd/compile/internal/gc.yyparse()
/home/ksh/work/os-code/go/src/cmd/compile/internal/gc/lex.go:1822 +0xb4
cmd/compile/internal/gc.Main()
/home/ksh/work/os-code/go/src/cmd/compile/internal/gc/lex.go:345 +0x1b35
cmd/compile/internal/amd64.Main()
/home/ksh/work/os-code/go/src/cmd/compile/internal/amd64/galign.go:127 +0x58d
main.main()
/home/ksh/work/os-code/go/src/cmd/compile/main.go:26 +0x189
ok crypto/sha512 0.003s
ok crypto/subtle 0.005s
ok crypto/tls 1.769s
FAIL crypto/x509 [build failed]
ok database/sql 0.048s
ok database/sql/driver 0.005s
ok debug/dwarf 0.015s

Not sure if it is related but also getting the following test failure (consistently):

ok cmd/fix 0.075s
--- FAIL: TestNoteReading-4 (11.84s)
note_test.go:46: buildID in hello binary = "", want "TestNoteReading-Build-ID"
FAIL
FAIL cmd/go 19.592s

Issue 2:
##### GOMAXPROCS=2 runtime -cpu=1,2,4
--- FAIL: TestStackOverflow (0.04s)
crash_test.go:92: building source: exit status 2
panic: runtime error: slice bounds out of range
fatal error: panic on system stack

runtime stack:
runtime.throw(0x985d70, 0x15)
/tmp/buildlet-scatch465833892/go/src/runtime/panic.go:527 +0x96
runtime.gopanic(0x891b00, 0xc208010190)
/tmp/buildlet-scatch465833892/go/src/runtime/panic.go:354 +0xbc
runtime.panicslice()
/tmp/buildlet-scatch465833892/go/src/runtime/panic.go:18 +0x50
runtime.gcInstallStackBarrier(0xc20863c9a0, 0xc208033c78)
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:510 +0xb1
runtime.scanstack.func1(0xc208033c78, 0x0, 0x1)
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:365 +0xa2
runtime.gentraceback(0x6286a5, 0xc2080fca18, 0x0, 0xc20863c9a0, 0x0, 0x0, 0x7fffffff, 0xc208033d90, 0x0, 0x0, ...)
/tmp/buildlet-scatch465833892/go/src/runtime/traceback.go:336 +0xa81
runtime.scanstack(0xc20863c9a0)
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:380 +0x351
runtime.gcphasework(0xc20863c9a0)
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:273 +0x70
runtime.stopg(0xc20863c9a0, 0x3)
/tmp/buildlet-scatch465833892/go/src/runtime/proc1.go:395 +0x91
runtime.markroot(0xc208018000, 0xd)
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:138 +0x1f2
runtime.parfordo(0xc208018000)
/tmp/buildlet-scatch465833892/go/src/runtime/parfor.go:95 +0x12d
runtime.gcscan_m()
/tmp/buildlet-scatch465833892/go/src/runtime/mgcmark.go:37 +0xe1
runtime.gc.func1()
/tmp/buildlet-scatch465833892/go/src/runtime/mgc.go:843 +0x5f
runtime.systemstack(0xc20801b500)
/tmp/buildlet-scatch465833892/go/src/runtime/asm_amd64.s:262 +0x7c
runtime.mstart()
/tmp/buildlet-scatch465833892/go/src/runtime/proc1.go:656

goroutine 21 [garbage collection scan]:
runtime.systemstack_switch()
/tmp/buildlet-scatch465833892/go/src/runtime/asm_amd64.s:216 fp=0xc2080204e8 sp=0xc2080204e0
runtime.gc(0x0)
/tmp/buildlet-scatch465833892/go/src/runtime/mgc.go:855 +0x1dc fp=0xc208020790 sp=0xc2080204e8
runtime.backgroundgc()
/tmp/buildlet-scatch465833892/go/src/runtime/mgc.go:775 +0x40 fp=0xc2080207c0 sp=0xc208020790
runtime.goexit()
/tmp/buildlet-scatch465833892/go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc2080207c8 sp=0xc2080207c0
created by runtime.startGC
/tmp/buildlet-scatch465833892/go/src/runtime/mgc.go:748 +0x14a

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc208617360)
/tmp/buildlet-scatch465833892/go/src/runtime/sema.go:43 +0x2d
sync.(*WaitGroup).Wait(0xc2086495a0)
/tmp/buildlet-scatch465833892/go/src/sync/waitgroup.go:132 +0x15d
main.(*builder).do(0xc2081399d0, 0xc208112d00)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1153 +0x3c9
main.runBuild(0xb90360, 0xc208056130, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:441 +0x51f
main.main()
/tmp/buildlet-scatch465833892/go/src/cmd/go/main.go:177 +0x750

goroutine 19 [syscall]:
os/signal.loop()
/tmp/buildlet-scatch465833892/go/src/os/signal/signal_unix.go:22 +0x1f
created by os/signal.init.1
/tmp/buildlet-scatch465833892/go/src/os/signal/signal_unix.go:28 +0x3e

goroutine 23 [syscall]:
syscall.Syscall(0x0, 0x6, 0xc2080fcae0, 0x8, 0x0, 0x8, 0x0)
/tmp/buildlet-scatch465833892/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.readlen(0x6, 0xc2080fcae0, 0x8, 0xf, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/syscall/zsyscall_linux_amd64.go:1085 +0x53
syscall.forkExec(0xc20800a880, 0x3d, 0xc20866e0e0, 0xe, 0xe, 0xc2080fccb0, 0x15d4, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/syscall/exec_unix.go:200 +0x56a
syscall.StartProcess(0xc20800a880, 0x3d, 0xc20866e0e0, 0xe, 0xe, 0xc2080fccb0, 0x1, 0x4, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/syscall/exec_unix.go:238 +0x6e
os.startProcess(0xc20800a880, 0x3d, 0xc20866e0e0, 0xe, 0xe, 0xc2080fcee8, 0x90a160, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/os/exec_posix.go:45 +0x33f
os.StartProcess(0xc20800a880, 0x3d, 0xc20866e0e0, 0xe, 0xe, 0xc2080fcee8, 0x0, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/os/doc.go:24 +0x6c
os/exec.(*Cmd).Start(0xc208670000, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/os/exec/exec.go:319 +0x76a
os/exec.(*Cmd).Run(0xc208670000, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/os/exec/exec.go:246 +0x34
main.(*builder).runOut(0xc2081399d0, 0xc208127100, 0x16, 0xc208127080, 0x17, 0x0, 0x0, 0x0, 0xc20866c000, 0xd, ...)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1902 +0x334
main.gcToolchain.gc(0xc2081399d0, 0xc208125400, 0xc208644b40, 0x30, 0xc20865e200, 0x34, 0xc208172600, 0xc20800e540, 0x2, 0x2, ...)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:2146 +0xf1e
main.(*gcToolchain).gc(0xbbcf70, 0xc2081399d0, 0xc208125400, 0xc208644b40, 0x30, 0xc20865e200, 0x34, 0x0, 0xc20800e540, 0x2, ...)
<autogenerated>:3 +0x191
main.(*builder).build(0xc2081399d0, 0xc208112d00, 0x0, 0x0)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1320 +0x2551
main.(*builder).do.func1(0xc208112d00)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1087 +0x3bc
main.(*builder).do.func2(0xc2086495a0, 0xc2081399d0, 0xc208649580)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1144 +0x158
created by main.(*builder).do
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1150 +0x3a1

goroutine 24 [select]:
main.(*builder).do.func2(0xc2086495a0, 0xc2081399d0, 0xc208649580)
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1134 +0x1d1
created by main.(*builder).do
/tmp/buildlet-scatch465833892/go/src/cmd/go/build.go:1150 +0x3a1
FAIL
FAIL runtime 25.490s

http://build.golang.org/log/de2894c6b3b2b342cb66bd859be2c197e8e641ad

/cc @aclements