While refactoring one day, I couldn't work out why my code had broken. That's not unusual, and so I checked and re-checked the new code that I'd written. Nope, it was bug free!
This was when I discovered the Anti-Bug Pattern, which occurs when there are two bugs in a system, but which co-incidently cancel each other out. Fixing one bug uncovers the other. Ruben Bartelink came up with the name.
Quantum / HeinsenBug
This bug only occurs when you are not debugging your code. As soon as you look at your source code... the bug disappears.