I made a little progress narrowing down the cygwin errors. If I simply run ./python.exe Lib/test/regrtest.py the first test which fails is test_bz2. If I run test_bz2 by itself it succeeds. I created a shell script which just executed pairs of tests, one of the ones which was run before test_bz2 followed by test_bz2. A couple tests caused test_bz2 to fail: test___all__ test_asynchat Here's the error output: C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 44 [main] python 1476 sync_with_child: child 1532(0x18C) died before initialization with status code 0x1 184 [main] python 1476 sync_with_child: *** child state child loading dlls C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 61438 [main] python 1476 sync_with_child: child 896(0x150) died before initialization with status code 0x1 61612 [main] python 1476 sync_with_child: *** child state child loading dlls C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 118777 [main] python 1476 sync_with_child: child 1452(0x114) died before initialization with status code 0x1 118944 [main] python 1476 sync_with_child: *** child state child loading dlls C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 208810 [main] python 1476 sync_with_child: child 1664(0xDC) died before initialization with status code 0x1 208994 [main] python 1476 sync_with_child: *** child state child loading dlls C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 294015 [main] python 1476 sync_with_child: child 1584(0xA0) died before initialization with status code 0x1 294200 [main] python 1476 sync_with_child: *** child state child loading dlls C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 371845 [main] python 1476 sync_with_child: child 1600(0x68) died before initialization with status code 0x1 372041 [main] python 1476 sync_with_child: *** child state child loading dlls test test_bz2 failed -- errors occurred; run in verbose mode for details 1 test OK. 1 test failed: test_bz2 [18388 refs] I then tried the same tactic with another crasher, test_fork1, and discovered these possible culprits: test___all__ test_asynchat test_cgi test_email There's the test_fork1 error output: C:\cygwin\home\Administrator\src\python\dist\src\python.exe: *** unable to remap C:\cygwin\bin\cygssl-0.9.7.dll to same address as parent(0x11B0000) != 0x11C0000 77 [main] python 1504 sync_with_child: child 1452(0x190) died before initialization with status code 0x1 212 [main] python 1504 sync_with_child: *** child state child loading dlls test test_fork1 crashed -- exceptions.OSError: [Errno 11] Resource temporarily unavailable 1 test OK. 1 test failed: test_fork1 [18201 refs] I then went back and tried test_cgi and test_email with test_bz2 (they don't run before test_bz2 in the normal case). Test_bz2 failed when run after either of them. In all cases with both test scenarios, the first test succeeded. I took a quick peek at the code of the possible culprits but didn't see anything which seemed to link them all together. Skip