Edward Cowling London UK wrote:

Several times I've been caught by this trick, so perhaps they watch for
it. Everyone hurtles up to the barrier and the person in front of you
offers up their ticket / card. They start striding through the barrier
and you follow up behind and put your Oyster on the reader, just as they
get the "seek assistance" screen. They carry on through (on your card)
and you get left there looking like a lemon.

Seems like this would be easy enough to deal with if the gate buzzed
upon an exception like this.
--
Michael Hoffman