successful troll
Trolls are people who start crap on the internet for fun, or 'the lulz.' But what happens when a troll targets, either knowingly or unknowingly, other trolls? An infinite loop of ridicule and posturing.

No one wins when trolls troll trolls! But when they turn against their own kind, I guess that means the rest of us have to deal with them less. Silver lining?

Learn more with video examples after the jump.
For the uninitiated, a quick definition of trolling from Wikipedia:
In Internet slang, a troll is someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking other users into a desired emotional response or of otherwise disrupting normal on-topic discussion.
We've all experienced trolling, if not directly.

Trolls trolling trolls occurs when someone intentionally baits trolls with an intentionally asinine or inflammatory remark. Then, in response, the counter-trolls will respond to the remark with faux disgust or outrage. Like this:

Troll 1: omg did you guys know taht glenn beck sin't even an American citizen himself? He was born in Canada! Bet you'll never hear that on Faux News!

Troll 2: wtf no he wasn't your an idiot he was born in Utah look it up on wikipedia if you dont believe me

Troll 1: LOLOLOLOL!!!!1 i can't believe you fell for that of course he's actually American

Troll 2: typical liberal. of course i knew you were joking it's called sarcasm idiot.

Troll 1: yah right

Troll 3: America is the dumbest country you don't even have free health care lol

and so on, forever and always.

Another example is when a troll backfires, as seen in the following video, wherein a guy calls into his local video game store to request a nonexistent game. Most of the people he calls are already privy to the classic 'Battletoads' troll, so none of them get upset. They just play along until he gets bored.

(It's supposed to work like this. Warning: The video at that link contains the foulest language you can imagine.)
Ugh, pathetic. Is there anything more sad than a grown man sitting in his mom's frilly living room engaged in a directionless prank that ends with confusion and regret? Just look at that Pottery Barn table in the background with the twin lamps. Tragic.
Know Your Meme did a great synopsis of troll behavior and a perfect example of trolls trolling trolls (aka cannibal trolls).
See also this classic illustration from 4chan.
Here's another. Don't let it happen to you. Sincerity 4eva.