Numerous times I have been on a train , a guard has discovered someone has no ticket or the wrong ticket and nothing has been done.
Examples that stand out in my mind include a 20 ish year old man who when asked simply said " I ain`t got one, what are you going to do about it" and the guard just walked on and continued checking other people`s tickets ( South Eastern) ; or on the other extreme, an attractive young lady who was on a peak hour train with an off peak ticket despite numerous announcements that it was a peak time train on the platform and the train. ( GWR) " Oh, I didn`t realise" whilst giving the male guard a sweet innocent cute sort of look and she was simply let off and not charged an excess fare. Having spent £30 more than her buying a peak ticket just to get that train instead of one that was £30 cheaper 20 minutes later ( I had to arrive at a specific time ) and being an unattractive old bloke, I cheekily asked the guard " would you have let me off if I had not had a peak ticket? " He didn`t reply.
I was actully much more annoyed by the second of those incidents than the first as perhaps I could more understand the guard feeling intimidated in the first one. For the second one, there were numerous loudspeaker announcements on the platform and train before departure that off peak tickets were not valid.