We had a bug recently where one of our internal bots didn't correctly parse usernames with hyphens in them, so instead of tagging @tom-jones, it would tag @tom
Talking about failing to parse names, I remember a co-worker once was working on a form and had added a special character validation field to the name field. The only problem was that it blocked apostrophes, and he was Irish so his form wouldn't actually accept his own name.