I've never looked into the details, but I imagine the paperwork Shark Tank entrepreneurs sign includes something along the lines of "all deals agreed are non-binding and subject to completion of satisfactory post-show diligence blah blah". I'd be shocked if a decent percentage of the deals featured don't ultimately fall through for one reason or another at the diligence stage with zero impact on the investor's reputation, so I don't think Cuban will lose too much sleep over it.