I've seen it happen several times at Aigburth. It is random. Last time I saw it at Aigburth a well dressed man got very irate because the revenue staff wouldn't let him through without a ticket.
"But I always buy a ticket at the other end!"
"Yeah right, you mean you never buy a ticket, but this time you've been caught!?"
I've seen people try it on at Liverpool Central where there are barriers. Sometimes they have revenue staff on the barriers and you have no way of getting through without paying a penalty fare; but the other day I saw a guy wave his bank card about and say something like "the ticket machine wasn't working". So the guy manning the barrier opened the gate and said "no problem, just go over to the ticket office and buy a ticket".
So he walks through the barrier and straight out of the station!