My favorite example of this is Tribes 1, players found if you tap jump fast while going down a hill, they could “ski” down and gain a lot of momentum. The dev’s hadn’t intended this to be the case, but players loved it, and the dev’s thought it was cool so they left it in. Now skiing is just considered a primary trait of Tribes games.

From what I understand this is also how the Spy in TF2 came to be.