[seiwyn]

People writing "loose", when they really mean "lose".

[fragmede]

Also: "payed" vs. "paid"; in grammar, "well" vs "good".

[Pengwin]

"your" and "You're" is my pet hate

[ciupicri]

Let's not forget about "it's" and "its".

[pjscott]

Apostrophe mistake's are pretty jarring; seeing one is like tripping over a rock, for me.

[kingofspain]

I can let any spelling mistake or grammatical murder slide except this one. I won't comment on it as that's a bit petulant, but I will seethe inside and often, almost without thinking, I'll disregard whatever else they wrote.

Silly I know!

[heycarsten]

Same goes for "alot" instead of "a lot".

[m-photonic]

Another one is "phased" in place of "fazed."

[mcknz]

a whole nother.