I've had bugs go away when I added `print("working fine until here")` to the preceding line. So if someone told me "this line is needed but I don't know why", I wouldn't even blink.

My coworker took down our site briefly by removing an html comment that said "don't remove this comment".

Load bearing print statement!

I've had a few of those, I think they're usually a symptom of stack corruption.

Or a concurrency/timing issue, where the delay of the I/O changes things just enough to make them work on your setup.

Back in the day the QA department would veto a release candidate of our firewall product due to performance degradation if a network cable in the testing equipment was bent too much during validation runs. Rearrange the cable and next test run would pass.

Timing matters.

This is also quite common in medicine...

Praise the Omnissiah!