Sniper posting 

Sniper posting is when you manage to reply just before someone else, therefore changing the meaning in the argument, or making someone else look stupid.

Pete: Your sister is so delicious
John: No, Andrew's sister is delicious
Andrew: I know, I got a taste of her yesterday
Pete: Nice sniper post, John.

In this example, John is writing his reply while Andrew is writing his, and posting milliseconds before Andrew. This is called sniper posting

Sniper posting by MisterNotSoT February 2, 2012