That's what I'd always heard too, c_b. Apparently no one knows for certain where the phrase originated, but one of the stories is that it was once a nautical saying. I think I mentioned on the list page that some of these may or may not have true nautical origins. :-)
Just for fun, here's a rundown of where people believe this phrase came from. I've actually read some others, too, while making my list.
I haven't heard that this was of nautical origin--actually only that it was said to (and of) printers' apprentices, who had difficulty telling apart the two letters in the type case (because they are backwards) and needed to pay careful attention in order to set the type properly.