About two years ago I encountered a bizarre bug where, if I asked SciPy for the eigenvalues of a particular small matrix that I was using as a test case, it wou

About two years ago I encountered a bizarre bug where, if I asked SciPy for the eigenvalues of a particular small matrix that I was using as a test case, it would consistently give a different result on my desktop computer than anywhere else. But when I tried to isolate the test case, it went away. It would only happen if I ran three other test cases in a particular order first. Or if I ran that one test case 12 times in a row, it would fail the twelfth time. I really wanted to find out what was going on. I looked through the code, from SciPy to ARPACK to the underlying ATLAS calls, at which point it became completely opaque to me. I still don't know whether it was the fault of ARPACK or ATLAS or what, but I just put the test cases in a different order, they consistently passed in that order just like they passed for everyone else, and a few system upgrades later the problem didn't happen anymore.