I find the fact that some barriers retain tickets quite an annoyance. The fact there is no rhyme or reason of which ones do or don't is annoying, as often you may want to keep the ticket for expenses or for delay repay, and have to ask a member of staff if it will get swallowed before you go through. Also surely the station operators have to empty and dispose of thousands of tickets every day, not to mention having to fish out ones that someone did want to retain, and also having to manually let through people at a manned gate. Seems easier to me to not retain tickets. There is a theory that some people would use the same day return over and over again every day though presumably?