I think my favorite example of this was in a game of Starsiege: Tribes, in the game's popular Shifter mod. Amongst its many changes, Shifter added in some seriously souped-up versions of the game's basic weapons, including the super-heavy tactical nuke. This was a weapon that could only be carried by the absolute biggest suit; one so heavy that the jetpack, at maximum burn, would barely slow down a falling soldier. Given the enormous map sizes in Tribes, you'd need to get someone to buy a transport and fly you over- and then, someone else to use a targeting laser to let you accurately aim the nuke, but it made up for these difficulties by not only possessing enormous destructive power, but actually (in this particular version, and only this particular version of the mod) had limited wall penetration.
So there I am one day, sitting on a hill near the enemy base beside my deployed radar jammer, sniping skiers out of the air, when I see a guy running in circles on a floating platform. He keeps it up long enough for me to realize he's using a targeting laser, which few enough people bothered to do that I figure someone on their team has a nuke. I aim toward the enemy base entrance and spot the heavy, standing stock still and taking aim. I don't think that he can hit our base from his, but, figuring it's best to disrupt him anyway, I draw a bead on his head. His suit is so thick and heavy that even a direct head shot from Shifter's most powerful sniper rifle won't kill him, but it'll either distract him and throw off his aim, or pull him away while he hunts for me and I can warn my team.
As I click the mouse, a muscle spasm runs through my hand, pushing the mouse over and spoiling my aim- or so I thought at first. Instead of sending a rail gun round through his head, I hit his weapon- apparently with such precise timing that it flew out of his hand just as it was firing (something I never even heard of happening in Tribes- before or since). I watched the arcing exhaust trail spin in a circle on the ground and wondered if the heavy even knew he'd been disarmed (losing a weapon in a tense, close firefight can be deadly; you can quickly switch to another, but you lose it unless you pick it up again- but the distances and speeds at which skiers tended to fight made this pretty much irrelevant) when the shell exploded, wiping out the entire defensive network of the base entrance and (thanks to that wall penetration) killing almost half the enemy team and destroying most of their base structures. My team, needless to say, won the match.
As an interesting side note, the poor heavy was immediately banned for teamkilling, having well exceeded the server's TK-per-time-period automated system. I don't know if it was a limitation of the technology or just the servers I played on or what, but in that time and place, bans were never temporary. Upon seeing the "[NAME] has been banned for teamkilling" message, I realized the downside of what I'd just done, and, once the match was over, went to the server-running clan's website to explain what had happened and plead the guy's case for him. He'd beaten me there, however, and, well, let's just say that I have never in my life been the recipient of so many scathing insults.
What are some of your most memorable moments of badassery that were either unintentional or unexpected? 
