Not saying it would happen with any major book, in fact i asked Slim this last year when i interviewed him, he did mention there were a few clerks who got caught doing things...I knew of a few who worked at a smaller book who did some underhanded things, I think the Book BetMega had a couple clerks who were giving away the shop, they went under..
I doubt any major book would run into this problem, just saying it is possible.
The Shrink had someone calling in bets under his account number according to him, and even accused the book itself.