Obviously in a system as large as Oyster there are going to be occasional malfunctions. Many of them, while annoying, will not have serious repercussions. If a gate stops working it will usually be blocked out of use, but what happens if the error doesn’t affect the actual gate function? I have become aware of three instances recently where Oyster data has been compromised in some way.
Back in the summer a single gate at London Bridge failed to update the date. Anyone using PAYG exiting through that gate ended up with two incomplete journeys, one ending on the previous day and one starting at their origin on that day.