[marcosdumay]

There's the alternative:

* The program works flawlessly

* Somebody finds a bug on the code that is executed in a test

* The relevant test now fails consistently

Both are probably because you changed something on the environment when you looked at the code.

[omoikane]

Another common cause is that the code you were looking at wasn't actually the code you were running. Between "I added this line and then I removed this same line, and now it works", the executable might have been recompiled.

[bobthepanda]

hot reloading can cause weird behaviors like this in particular