What is immensely frustrating is to aim to arrive about 10min before, then be held up by factors such as tube/bus delays and reach the barrier a minute or so before advertised departure time. The train is there, you would have enough time to reach the first door but the barrier staff refuse to let you try.
I'm aware of all the reasons for this but my point is that it makes passengers very angry either with the railway in general or with the so-called jobsworths who run the barrier, even though they are enforcing a policy set by some faceless manager somewhere who never has to endure the comeback. Count the above double or triple if you have an Advance ticket and the company benefits from your misfortune to the tune of a walk-up ticket for the next train.