Rusty programming

I spent literally hours trying to establish why adding some 'sanity checks' (for the existence of files on a first-time run) to my AO3 stats script had managed to break the existing functionality.
Eventually it turned out that the significant change was that I had accidentally added a couple of blank lines into the input data I was using to test with, and not anything to do with my improvements to the actual program at all!
(Arguably the software should be more tolerant of blank lines in its input...)