Google announced on Google+ that they can now automatically spend twice your average daily budget in order to help you reach your goals. This will not result in you spending over your monthly budget, but it can mean your daily budget is doubled on some days and a lot less on other days.
Google wrote:

Starting October 4, 2017, campaigns will be able to spend up to twice the average daily budget to help you reach your advertising goals, like clicks and conversions.
On days with lots of high quality traffic, your costs could be up to 2 times your daily budget. This spending is balanced by days when your spend is below your daily budget.
Keep in mind, you won’t be charged more than your monthly charging limit: the average number of days in a month (30.4) multiplied by your average daily budget.

I assume Google was missing out on monthly spend because of the daily budget limitations and now Google is able to fully use everyone’s monthly budgets this way.
Not sure if all advertisers are happy about this but I didn’t see too much chatter around this change yet.
Forum discussion at Google+.