I've had some inaccurate spendings in the past, but nothing more than 10%-15% a day.
But today, my adwords budgets finished arround 2:00pm pacific time!
fsichermx, this may or may not pertain to your situation, but here is the most most common reason for a budget to be fully spent early in the day:
The campaign uses very competitive, high dollar keywords (often with a really high Max CPC too), in conjunction with a 'too-low' daily budget that would be more appropriate for fewer (and less competitive) keywords.
AWA