One of the longest running campaigns I have been involved in featured a ranged weapon expert ranger and a fighter. While the fighter charged up into melee, the ranger would typically hold back and launch a few arrows into the fray. Well eventually, a few arrows would hit the fighter resulting in a gruff comment from the fighter's player and laughs from the other players and DM. Then it continued. Eventually, it appeared to everyone that the ranger was taking shots that had a greater chance of hitting other party members on purpose. The ranger would actually move and adjust his firing arc to ensure that a party member gave his target cover. This behavior was never explained, either by the player or the character.
In the same campaign, a sorcerer (played by an experienced player) launched a fireball through a web entangling the party. Or, rather, attempted to. The web ignited and everyone took extra damage from it cause the fireball couldn't make it through the several layers of cover.
I still game with the same people (new campaign). The table has a legend now that "accidently" doing damage to fellow party members earns extra exp. Of course, the sorcerer's player is now the DM...