I've had this happen a couple of times in an Uber cab, where it is easy to notice. In those cases it was clear from the driver's reaction that he just forgot, and I asked him to add a couple of dollars to the final meter.

I've never had this happen in a regular Uber, and I think it would be more difficult to deal with there. The drivers can't alter the amount billed (at least, I don't think they can) and I sometimes don't carry cash when I'm taking Uber.

I've also had the situation happen where an Uber X driver kept the meter going for about a mile after he dropped me off. A quick e-mail to the company fixed the issue. In general, I don't take Uber X now because the drivers are not always very good and the quality of the car is often questionable.