I was capturing QuickDraw library calls - the low level graphics primitives, to figure out where the graphics time in apps was going and found out sometimes excel did it 9 times
Of course users didn't see it more than once, but our hardware made all that wasted time run faster
Another dev who's fixing a bug, realizes if they call a certain function either directly or indirectly, their particular bug gets fixed.
Oh, and as a side effect, the cell gets erased (again).
A few more fixes/new features added like this and the code is inadvertently erasing the same cell multiple times.
It takes a certain type of dev to step through in a debugger and Notice the app is doing way too much work and then to untangle the mess of code without causing regressions.