The majority of glitches, errors and other goofs found in video games are not intentional whatsoever. Ubisoft sure didn't intend to embarrass itself with Assassin's Creed Unity's downright creepy floating eyes bug, and Warner Bros clearly didn't plan for the PC launch of Batman: Arkham Knight to be a completely broken mess.
These examples - alongside many, many others - illustrate just how difficult game development is, because even with four years of hard work by hundreds of talented artists, things can (and do) go horribly wrong.
Because these glitches are such a common thing for gamers to deal with, it's not unreasonable to brand even the tiniest of problems as an oversight or an unacceptable error. In reality, some of these "mistakes" are either left in or planted there intentionally by the developers, whether it's to give the player something cool to discover, or just a funny little quirk that nobody thought was necessary to change.
After gathering one batch of these totally-done-on-purpose gaffes and screw-ups, we received a ton of additional ideas in the comments, and so, we decided to gather those up too. Here's what you picked!