I have to ask.

[Turtle Tamer Kammy]

What the heck is up with all the "Not so fast! ******* is serious business!" signatures?

 

[Rayne]

They're called internet memes; sayings that become popular and such and
get used over and over again. Think of it as a small fad.

 

[Nekusagi]

And it's just plain funny. (BTW, it's a Yu Gi Oh reference)

 

[Misty]

Actually, it's a combination of Yu-Gi-Oh ("Not so fast, Kaiba! You've activated my TRAP CARD!") and another internet meme ("The internet is serious business").

 

[Nekusagi]

Ah. I knew PART of it was YGO...