In the late 1990s my friend was writing a game for his TI-83 calculator in TI-Basic. He was running into this bizarre bug we boiled down to a single IF after almost an hour of back and forth over a single calculator. The IF was not behaving as you would expect and it made zero sense. In the early version of TI-Basic, operators are actually single symbols, rather than made from text characters. In frustration I delete the IF symbol, insert a new one, and fire the game up. Everything works, and my friend just about dies in disbelief. It's probably my most frustrating bug fix.
I was telling someone the story a couple years ago and they said the opcodes linked to the symbols could get corrupted or something like that.