Once it could be due to your unexpected site maintenance. Probably you never heard about it because you have great hosting. Are you sure?

Another time your daily budget was overspent 200% and you have to think how to explain it to your customer.

Yet another time somebody forgot to mention that part of website has been reworked and your ads are pointing somewhere … but it is for sure not a proper place. Probably you heard about broken links.

At the end good news, you had a good return for one of your products - even you increased a bids and budget to get more sales. And hurray! You sold out everything! But realized too late to narrow the budget when your products will be out of stock and half of your budget were spent for nothing.