Way too many to count;

One of the worst was on another site during an elder scrolls rp; 50 kg vampire woman (who didn't burn in sunlight) and who was the most attractive woman in the world, who killed an entire team of trained imperial soldiers with a massive summoned daedric greatsword and didn't even get a scratch :rolleyes Guess who the GM was...

Other examples include an NRP where a players made a god intervene to deus ex machina their way out of a war. 

I have some GMODrp examples too. In a fallout roleplay someone powergamed my character and beat him into a pulp (Their character was 16 and had stumbled into our base wounded from bandits, my character was signed on with the NCR and had been saving his life at the expense of our own medical supplies; Their only motivation was to 'rob my gun', :| ) and when I was playing hl2rp as a DvL (for those who don't know, a DvL is a high ranked commander in Civil Protection and is augmented to shit, ie; they are supposed to have fights tilted very much in their favour) in which a malnourished citizen dodged all of my attacks and all the attacks of two other units in cqc, power gamed knocking my stunstick out if my hand, taking my 9mm off me and shooting me in the face all in one action. Unsurprisingly, an admin was called in and took our side rofl. Also in hl2rp, the city got attacked by antlions (being roleplayed by trusted players who we had model swapped) and one of the citizens ran out to the antlion guard and did /me jumps onto the aliens head and kills it *facepalm*