I found them to be less than informative when I used an oyster for the first few times years ago. There was all this information about having to touch in and out, but the instructions were quite ambiguous. Wimbledon has an entrance for overground and tramlink. So I touched in twice then touched out again at the end of the tram journey. I got charged for two journeys. I queried this and was sent a uniformed letter explaining the procedure. Because of the nature of this circular letter it told me that many more people had been confused the same way, maybe 1000s. Sneeky or inefficient? Who knows.
What is ransomware and how do I protect my PC from WannaCry?