It happens like this: you can pay rent without penalty up to say, the fifth of the month. Auto pay is set to come out of your account on the first. Sometimes you need the extra few days to get everything in line, but because everything is so tight and hectic, you forget that you won't actually get to flex those few days, and the payment comes out earlier than you expected.

The timing of payments can be a real burden when you're on the edge. I wrote about a time something similar happened to me a few years back https://news.ycombinator.com/item?id=6424493 This is about overdraft fees, specifically, but it's the same principle.