Well it comes down to you essentially flouting/breaking their laws.

So this then depends on the border guys dealing with you.  If you had a valid reason for it (heck, you were kidnapped, insanely ill, etc) and can prove it, you may be ok.  Or they may be feeling kind.

On the other hand, they may decide as you've broken the law, there's punishment.  When I've seen this happen in other countries to travellers I was with, they were interrogated, and then fined (and that was only by a few hours!)

Worst case - you might get a black mark in your passport or on your record, which may cause you problems when you try to visit other countries.

Long story short, it's almost never worth flouting these laws.