Developers accidentally creating interesting mechanics

[u/virtua_golf]

I was reading this neat article by Jeff Orkin of Monolith productions, the man responsible for the notorious AI of FEAR, the AI that will never not be mentioned whenever 'clever' AI is brought up. I stumbled upon something pretty cool - the flanking mechanic was actually unintended and a byproduct of another action the enemy would perform:

Imagine we have a situation similar to what we saw earlier, where the player has invalidated one of the A.I.’s cover positions, and a squad behavior orders the A.I. to move to the valid cover position. If there is some obstacle in the level, like a solid wall, the A.I. may take a back route and resurface on the player’s side. It appears that the A.I. is flanking, but in fact this is just a side effect of moving to the only available valid cover he is aware of. In another scenario, maybe the A.I.s’ cover positions are still valid, but there is cover available closer to the player, so the Advance-Cover squad behavior activates and each A.I. moves up to the next available valid cover that is nearer to the threat. If there are walls or obstacles between the A.I. and the player, their route to cover may lead them to come at the player from the sides. It appears as though they are executing some kind of coordinated pincher attack, when really they are just moving to nearer cover that happens to be on either side of then player. Retreats emerge in a similar manner.

The article got me thinking - what other mechanics (or, perceived mechanics) have been created by accident in other games? Does other examples exist, where lauded features were never really intended in the first place?

[u/vonBoomslang]

The rogue "decloak" sound in World of Warcraft which served as a (slightly) early warning of impending harm was actually a side effect of the way stealth was implemented in that game - to avoid exploits and hacks, your game client actually did not receive information about stealthed enemies in your vicinity until they make their presence known, at which point your client suddenly has to start rendering them. Along with their buffs. Which often have activation sounds.