8. Dumpster Grenade - Half-Life 2: Episode 2
In a great example of developers having to keep themselves entertained as well as the end user, this is something of a "non-accidental accident".
When players of the fantastic Half-Life 2: Episode 2 came across a big, American-style dumpster which appeared to house a head-crab zombie, the obvious course of action was to chuck in a grenade to take care of the creepy critter.
Imagine players' surprise when the grenade was thrown straight back out again...
Yep, that's right; in an entirely unexpected turn of events, the head-crab zombie does what more enemies should probably do and launches the grenade right back at the player.
The story behind this iconic moment was revealed by programmer Kerry Davis in the game's commentary track, and is actually quite a straightforward tale, it turns out.
It transpires that the devs were just as surprised as the players when they first stumbled upon this grenade-tennis as it was a total accident, a glitch as opposed to an intentional, scripted event.
Finding it hilarious, however, the developers took this complete fluke of a moment and actually programmed it into the game's final code.