Here are a couple of enormously annoying mistakes. First, as my background before Perl (B.P.?) was C/C++, I always had fun forgetting to prefix my variables with $ or better yet, getting careless with Perl's flexibility and not initializing some variables before they were used.

Second, this is more to do with the keyboard I use but it affects most of my work which is in Perl. I have a British keyboard and the British Pound happens to be next to the $ and occasionally my finger slips. Through telnet, I was always accidently placing pounds and whenever the scripts executed, I always got something along the lines of '\024 not found'. The pound never showed up. Only by using a windows program, UltraEdit, did I catch the mistake. Hugely embarrasing.
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"I slept with Faith, and found a corpse in my arms on
awaking; I drank and danced all night with Doubt, and found
her a virgin in the morning." Aleister Crowley, _The Book of Lies_