A friend of ours, who had rsvpd with no +1, reached out to us the week before the wedding to ask if he could bring his brand new girlfriend. We said sorry, no, we've already had to finalize the seating charts and headcount etc. (And also we literally have never met this girl). Well of course he still shows up with her, late to our ceremony so they're awkwardly standing in the back the whole time. At the reception they stole one of my groomsmen's chairs from the head table in order to squeeze her in at their full table. Thankfully the venue staff were great and didn't bat an eye, just brought an extra chair and an extra meal no questions asked.
Also my ex (who was invited as he's still part of our mutual friend group) showed up in pants that were way way too tight. Proceeded to bust a move on the dance floor and rip the entire a*s crack of his pants. One of my bridesmaids brought him into the ladies room and sewed him up using the emergency swing kit I'd put in the bathroom basket. But that one was more hilarious than annoying.