The normal event of accruing more costs in one day than the amount of your average daily budget. However, Google Ads prevents your campaign from overspending for the month as a whole.

• Overdelivery can help make up for days when traffic is slow and your ads don't get as much exposure. Here's how: to make sure that you don't miss showing your ads on a popular day, Google might use more of your budget on some days and less on other days. When this happens, your total daily cost could be up to 2 times your average daily budget when you aren't paying for conversions.  If you are paying for conversions your daily cost could be more than 2 times your average daily budget.
• However, our system makes sure that after an entire month of service, you're never charged more than your monthly charging limit -- the average number of days in a month (30.4) multiplied by your average daily budget.
• If Google overdelivers your ads too much, and you accrue more costs in a complete monthly billing period than your budget allows, a credit might be applied automatically to your account.
• Note that for campaigns that are paused in the middle of the month or that otherwise don't run for the full month, you may see discrepancies between your average daily budgets and your total charges.

More about overdelivery
More about monthly charging limits
See articles about managing your budget

* Nguồn: Google