In Coders at Work, Joe Armstrong cites Joe’s Law of Debugging: all errors will be plus/minus three statements of the place you last changed the program.
(Read the entire page here.) If I had remembered this last night, instead of this morning, I could have saved myself 3+ hours of debugging.
Note to self: next time you’re about to start debugging — especially after 23:00 — for the love of God, first check the diffs of the last couple of commits!
This is just one of many nuggets of wisdom in this excellent book, which I recommend highly.