Don't risk doing this and just expecting that CBP won't care next time you try to enter. Go to the CBP office at the nearest airport or the nearest USCIS office and try and get them to give you official permission to leave later.

They seem to have been authorized to offer VWP travellers "satisfactory departure" if they leave within 30 days after their 90 day limit, but it's not automatic: it seems they need to stamp or write something on your I94-W receipt.

Similar measures seem to be available for visa holders, but for them it's easier anyway to apply for an extension of status through USCIS. But again, it must be done actively and not just assumed to have been granted.