There is a thread at WebmasterWorld that discusses a flaw in the adword budget allocation system that affects mostly European sites. As cline said in his post:
Here's the problem: Adwords allocates daily budget on an hourly basis using PST.
If you're running a European-targeted campaign, and especially if that campaign is B2B with the vast bulk of the traffic coming during business hours rather than the evening, you're going to quickly blow your hourly budget.
Doug Bates of Aderit Internet Marketing Consulting brought this to my attention.