I remember once at T1 (probably around 2008 when bmi used to have a route network that was more useful to me) the ticket office member of staff claimed that there were not able to sell Heathrow Connect tickets and also tried to claim they couldn't issue discounted tickets (Network Gold Card in my case).

Going back further (pre-T5) the last train on a Sunday night used to be a Heathrow Connect, after checking with the ticket office that claimed the last train was definitely a HEX and sold me an appropriate ticket only for me to get to the platform and find out the only train leaving the station now was the connect.

Why did I encounter staff like that? Are they on commission? You never see behaviour like that at true National Rail ticket offices.