I've been a First Class season ticket holder for a few years, on Southern/GatEx, and have therefore overheard many passenger conversations with RPIs/Conductors.
I think the top three excuses by number have been:
3. I didn't know it was First Class.
2. I have a bad back/am feeling ill, and there weren't any other seats.
1. There weren't any other seats.
When caught, the 'I'm only going one stop' line is often wheeled-out, which strikes me as a fairly feeble excuse. Certainly, people seem to feel hard done by when checked by RPI between Victoria and Clapham Junction, as though that short hop should be a free-for-all.
One novel 'excuse' I heard was 'I don't believe in the class system'.
I sympathise with revenue staff when they encounter pregnant ladies, or those claiming to be pregnant. There is a Priority Seat scheme in Standard on Southern, but I've seen a few with 'baby on board' badges who make a beeline for First. They are generally allowed to stay, with words of advice about the Priority Seat scheme, or otherwise asking the Conductor before taking a seat in First. (I do not, by the way, think heavily pregnant ladies should have to stand on busy trains, before anyone goes on the attack.)