The TfL contactless system doesn’t generally overcharge, as long as you properly tap-in and tap-out.
The system as a whole seems to be designed to prevent overcharging at all costs. In situations where is ambiguous what you should be charged (there’s a number of interesting edge-cases), the system always charges you the lowest possible amount. Even in the event of user error (such as forgetting to tap-out), the system does it best the guess what you did, such as looking at your history and assuming you meant to tapped out at a station you normally use, or looking for a station where the gates where left open (due to a major event, overcrowding concern, evacuation etc), and assuming you left the system there.
The contactless system is substantially more capable for handling these issues correctly, and generally better at charging less, than the older Oyster system.
The system as a whole seems to be designed to prevent overcharging at all costs. In situations where is ambiguous what you should be charged (there’s a number of interesting edge-cases), the system always charges you the lowest possible amount. Even in the event of user error (such as forgetting to tap-out), the system does it best the guess what you did, such as looking at your history and assuming you meant to tapped out at a station you normally use, or looking for a station where the gates where left open (due to a major event, overcrowding concern, evacuation etc), and assuming you left the system there.
The contactless system is substantially more capable for handling these issues correctly, and generally better at charging less, than the older Oyster system.