Incomplete journeys will be charged a maximum fare (which is going to be £5.80 on this occasion), not a penalty fare (£80 or £40 for prompt payment). Maximum fares will generally be refunded automatically within 48 hours if you only have the odd one every so often, or you can email TfL to get it refunded up to 3 times a month. Sometimes it is not possible to touch out, for example at Chelsea home games when the other exit to Fulham Broadway is open, you are pushed forward by the crowds before you have the chance to get anything out of your pocket/handbag; and staff have sometimes even shouted "do not touch out".

Penalty fares require you to be stopped by a revenue inspector and your details taken, which is not going to happen during travel to a crowded event, although as I said, Chiltern has recently been increasing the presence of revenue inspectors at Marylebone to catch out doughnutters and those buying railcard-discounted tickets while having no railcard, or an expired railcard.