That if Standard Class is full, you can sit in First Class for no extra charge.
To be fair, this is something people may have got from airlines.
As it's not unusual for economy to be overbooked and to get bumped up to business class FOC (it's happened to me a number of times).
And, arguably, from a consumer's perspective (knowing little about how railways work) railway operators should do the same.
As a very young enthusiast, I could not get my head around the idea that a named train did not have the same named engine.
For example, travelling on 'The Master Cutler' and 'The Clansman', which were hauled by engines with different names.
I presume many people probably think this when they hear of 'The Flying Scotsman'.
That the diesel or electric motive power unit that heads a train is an engine. It's a locomotive. The engine is the prime mover inside a diesel locomotive (or on or under a DMU car).
Yet many do use engine instead of locomotive, in my case probably through laziness at not wanting to use extra syllables.
In economics/business, many people similarly use 'market' rather than industry, when sometimes an industry does not have competitive demand and supply (and therefore is not really a 'market'). And again, some of this is just laziness by people who actually know better.