The return portions of tickets from south Wales to Brentwood or Shenfield never seem to work at Liverpool Street. Back in class 315 days I used to go out to Shenfield in the evening and then back to Brentwood for the Premier Inn. Then take a pair of 315s up to Liverpool Street the following morning. I didn't put the return portion through the gates at the start of the journey as it was routinely rejected so I just asked the staff at Brentwood to let me through. Perhaps that is what confuses the gates at LST? Anyway, the staff at LST were always happy to let me through, apart from one grumpy guy who put the ticket through twice and concluded from the error code that I was somehow trying it on. Nevertheless, he eventually let me through.My tickets at Liverpool St get rejected at the barriers all the time, but the staff have never been rude to me, they just grunt and let me through.