Saying "what" instead of "that",
or worse, writing it. I've even seen it on official notices in Tube stations.

One that baffles me more than annoys me, writing "then" instead of "than". Common and excusable enough when English isn't the person's first language, but I don't know how anyone else can get these two words confused.

Now some people have a bee in their bonnet about whether you should say "different to" or "different from", but the one that really gets my goat is the (especially American) habit of saying "different than".
( Moon Girl Technologies horrendous beanbag, Sat 10 Apr 2010, 14:00, 2 replies)

Different from
differs from, and similar to.
( sandettie light vessel automatic New Twitter - @bollocksreally, Sat 10 Apr 2010, 15:01, closed)

See I don't think it's as simple as that,
correctness shouldn't be about strict application of arbitrary rules; the purpose of grammar is to convey meaning, not to demand obedience. Sometimes I say "different from", and sometimes I say "different to", depending on the context; just as both "come from" and "come to", and both "go from" and "go to" can be correct, depending on what it is you are actually saying.

Always "differs from", though. I can't think of a single context in which "differs to" would make any sense.
( Moon Girl Technologies horrendous beanbag, Sat 10 Apr 2010, 15:11, closed)