Malcolm & Nika wrote:

Sensors on the gate stop someone putting in another ticket until the first
has cleared the sensor. The non ticket person pushes up against the gates
and this clears the sensor (didnt take them long to work it out)....so the
next ticket is able to go in. Sometimes the non payer reaches around and
takes the good ticket...leaving the second person saying "my ticket is lost
in the machine". The gates actually open when you take the ticket out, not
put it in.
The answer is not to take out your ticket until the person in front is gone.
But in the rush......this is a hard one to remember.

Even easier to get in when the person behind you has an Oyster (no
"taking the ticket out" to be done).

On the occasions it has been done to me, I don';t think it was malicious
- it is usually someone whose ticket said Seek Assistance, but they
didn't notice.