When you type like a complete idiot.

"Goin 2 da mall 2day 2 buy sum clothes. Hmu if u want 2 cum C me."

Oy. How can you honestly expect anyone to take you seriously? Yes, of course, this is just the internet and not English class. The problem here is that your grammar is the impression you leave behind on the internet. Pretty much everything is online these days. In a world where everyone emails, texts and communicates through social networking sites, it's important to take your internet grammar seriously.  It takes possibly 15 more seconds to type and spell a word correctly. And for shit's sake, use punctuation.

Honestly, you look both lazy AND stupid when you fail to use proper grammar. Even if you misspell a word, or use the informal and simplified versions of longer words, who cares? At least you put forth an effort to type out a normal, understandable statement.