When I was a larval programmer, I collected the different types of bugs I found as I debugged my code. This category was my favourite.
I once progressively commented out more and more of a file, trying to find the bit with a bug, until the whole thing was commented out and no source was being compiled yet the original buggy behavior was still there and only THEN did I figure out I was editing a different file than I was compiling.
Echoes of this original sin have shown up repeatedly throughout my life, to be point where it's one of the first things I check when my corrective measure has no effect on problematic behavior.
I once progressively commented out more and more of a file, trying to find the bit with a bug, until the whole thing was commented out and no source was being compiled yet the original buggy behavior was still there and only THEN did I figure out I was editing a different file than I was compiling.
Echoes of this original sin have shown up repeatedly throughout my life, to be point where it's one of the first things I check when my corrective measure has no effect on problematic behavior.