[danpalmer]

I suspect a factor that makes this trickier is cost per click. You're bidding for a click not a particular spend, so if your cost per click is ~$0.01 then overrunning by 100 clicks only costs $1 over budget. If you're more like $1-5 a click (not uncommon) then you can easily get to $100 over budget.

My guess is that this is worse the lower your budget, and $100 a day is a low budget for Google ads when you're considering the whole industry.

I would expect that with a budget of, say, $5000 a day, you might still be ~$100 out.

[heavenlyblue]

No. They overshoot because there’s a small-letter asterisk within the budged definition that allows then to overshoot at twice the amount.