Lots of these examples aren't really grammatical mistakes. xD
I know that most of you were doing it for the comedic attribute, but I was just confirming just in-case someone was serious.
In terms of grammar, I often see lots of them in lobby, hub, etc.
I can't really remember the "worst grammar" that I have ever seen, but the most common one and frankly the one that bothers me the most is the "Your" vs "You're" mistake.