I remember there was a glitch with the Dual Holster that made it so if you wanted 2 guns with 2 shots each (A total of four shots) every time you went through the loading screen you second gun would turn into the very first pistol you find in the game (Making it so that you have to Different guns and it was a total of 3 shots).
I havent played much of AC3 since december so i dont know if they ever patched it. I know they completely ignored this glitch in the "ThanksGiving Patch"
The worse for me was the Homestead glitch where everyone disappears after the game. Even the Thanksgiving Patch didn't fix it and I nearly gave up on the game. When I went for the Prince of Thieves after the Holiday Patch and noticed they were there I started playing single player.
I never crafted in AC3 so I got nowhere close to the Dual Holster and the fact that I didn't upgrade my crafters due to rushing the game cuz all the ending leaks. I never actually ran into anything game breaking. Though I hated seeing Connor's model jump up for a split second. It made my eyes jolt and I got the sense that it hurted my eyes. Doesn't help that I have jaggies and artefacts all over the game. But that was then...
That said, get both patches if you want to dramatically improve your experience.
Nothing is True, Everything is Permitted I shall forever hate the name......Crimson Viper