I had an incomplete journey transferring from tube to tram at Wimbledon. I asked a member of staff for assistance as I couldn't see a tap out post. He told me to just tap in on the tram.
It showed as incomplete journey, assumed tap out Wimbledon. It charged me on that assumption.
AIUI▸ many staff members at Wimbledon just don't understand how it works. There is no way to touch in 'on a tram' if that's what he really meant.
There are two sets of validators on platform 9/10, alongside the tram stop positions, and those facing you as you walk towards a tram complete your rail or tube journey and start a tram journey.
There's a 'third party' explanation here:
http://www.oyster-rail.org.uk/wimbledon/Paul