I'll second the "watch new hires carefully".

You can save yourself a lot of problems if you just keep an eye on your new people for the first month. Obviously you don't need to be overbearing, but at the end of that first month you should do a detailed work evaluation (code review if a programmer). If the employee shows signs of producing crap in their first month, get rid of her/him then. Do not wait unless there is a really, really good reason to.