In message , Clive D. W. Feather
writes

The first design of gate involved you walking through the gate while it
processed your ticket. At the end, you were either released with your
ticket or dumped into a "sin-bin" for an inspector to come over and
investigate.
I seem to remember the first ones had two sets of gates but the second
set only opened if your ticket was valid, thus trapping you, however
these were quickly modified when it was found they trapped innocent
heavily pregnant women because the first gates closed before the second
lot opened
--
Clive.