[Adverblessly]

> To me, the hardest bugs are nearly irreproducible “Heisenbugs” that vanish when instrumentation is added.

A favourite of mine was a bug (specifically, a stack corruption) that I only managed to see under instrumentation. After a lot of debugging turns out that the bug was in the instrumentation software itself, which generated invalid assembly under certain conditions (calling one of its own functions with 5 parameters even though it takes only 4). Resolved by upgrading to their latest version.