Well, here I am! Ian's invited me to contribute the odd article, so I thought I'd start my flagging up something that I noticed yesterday in an old science-fiction novel I happened to open more or less at random. A character is explaining the role of a tank:
"The light tanks are used to scout out enemy positions and cross difficult ground. Its armour is weak, but the weaponry it uses makes up for that."
This is the kind of bad English your eye doesn't quite pick up immediately. I reached the end of the second sentence aware that it wasn't right somehow, and I had to read the quote twice to realise exactly what was wrong. It's that the speaker has switched from a plural as the subject of the first sentence (light tanks) to a singular subject in the second sentence (a light tank), without telling the reader that he's changing the number of tanks being discussed.
So what's so bad about that? We can be pretty certain from the context that the subject is "light tanks in general" throughout the quote. What this does, however, is slows down the eye. Either consciously or not, the reader notices that something in the writing jars, and either goes back to check that he read it correctly, or continues with less certainty than he had before. In either case, the end result is that the writing is weakened, and by slowing the reader you're in danger of losing his concentration and interest in what you've got to say.
Some mistakes make the writer look pompous or stupid; this one is more subtle, making the point you want to put across slightly harder for the reader to see. Little things like this add up easily.