Democrats getting comfortable about Hillary's good polling numbers would create a more likely chance of there being a major mess up that could cost her the race.
There might not be a major mess up, but any minor gaffe, any bad poll, any sign of friction between campaign staff is going to be blown up as apocalyptic events. The beltway press still call her book tour a DISASTER! due to one gaffe and the scrutiny is now going increase a hundredfold. She's going to need a strong strategy in place to deal with it.
