Glasgow where the guy pointed out that a return ticket to Mallaig was cheaper than the single I'd asked for.
Dublin where the clerk asked if the two of us were married? Anxiously, we told him "No" (this was 1980's Ireland), He said, "Look, say you are married and I can sell you a 'Family Ticket', it's cheaper". He sold us a "Dog, Bicycle or Pram" ticket for the tandem too.
Just today, in Cheltenham, I went to buy a ticket for a trip to Exeter. The guy cheerfully offered me a ticket to Exmouth via Exeter for £6.50 less.