Beware the ticket machines at Heathrow T4

The ticket machines at Heathrow do not sell the full range of tickets and (example from today) do not offer the off-peak tickets correctly.

For my journey the only fare it offered at 12:00 was a Anytime Single when the correct fare was off-peak, a difference of £15 before railcars discounts. The only advice the staff at the ticket machines could offer was to buy online and collect, they were aware that the machines did not always correctly offer the cheapest fare. They agreed that I should have been able to buy the off-peak single I bought online, but had no way to get the ticket machine to issue it directly.

If you didn't know what you were expecting then it would be easy to overpay.