So, I switched my cell phone provider last month. When my last bill came due, I explicitly checked with Sprint to see if said bill was for past charges or future charges, because I didn't want to overpay, and if I could wrap the whole thing at once, I would.
They said the charges were for expected monthly usage, that I shouldn't pay my current bill yet, and that the next one would have the final amount owed. You can imagine I was somewhat surprised when my next bill included the entire amount of the previous bill, plus a late fee.
It's been resolved, but I have to wonder if this was just an innocent mistake, in that they legitimately forgot to add the credit to my account that they explicitly told me I would be receiving, or if they deliberately leave these errors in place on the off chance that customers just reflexively pay their bills without looking at them. If I hadn't made that initial contact to figure it out, I may very well have done that.