Had to jump in with something that drives me mad. I can't say if I've seen it here or not, but I've seen it for years on other web forums and bulletin board systems.
For a long time I've noticed some confusion between the words "then" and "than", resulting in sentences like "Mine is bigger then yours".
I'm convinced there is a teacher somewhere who is confused or can't pronounce correctly. This produced scores of individuals who use "then" instead of "than".