If you tell someone in a ticket office what you want to do, and they sell you a ticket for this then that ticket must be valid. And by law, it IS. Sadly the law doesn’t apply to the railway in real world terms and we do see passengers following the instruction and/or advice of a member of “The Railway Family” being prosecuted for doing so. The necessity to not dob in your colleague is a far greater priority than a passenger keeping their job as a Barrister so “they told me to” is actively ignored.
"The man on the platform said..." is one of the age old problems. Personally I know many railway staff are useless at doing the simple things like making out an endorsement note. I personally carry a little A6 notebook with tear out pages that I use to put the requisite details on for anything I say.
However, it is also a favourite of chancers so I tend to look at the balance of probability of what they've said. TOC only tickets or advances on the wrong TOC - they may have asked someone if this is the train to say York but not shown their tickets. Busy member of staff says "yes", on they get and "the man on the platform said it was my train". They've not asked if it's their train, just where it is heading to. Less sympathetic where it is spelled out in detail on the booking and their ticket with a time and train operator saying "specified train only".
If there is disruption, many station staff or traincrew will assume that ticket inspectors will show their own latitude or common sense approach to restrictions. I've caught staff in the past while hanging around information desks telling people to board trains without formal ticket acceptance and in one instance the station announcer at Man Picc boomed to the whole station that passengers off a cancelled TPE service could use my train - which formally, we hadn't agreed. I of course gave the announcer a call and told them to pack it in ("we thought it would be OK") and passed anyone who did board. Consequently any disruption as a matter of good nature I do pass any form of ticket regardless of whether acceptance is in place or not.
I came across a stunner the other day - two passengers had paid on a TVM for walk up tickets for the thick end of 70 quid. The message "tickets failed to print - contact booking office" had come up but the office was closed due to staff shortages. They said they'd spoken to the station supervisor who had told them to board the train and the booking office at their interchange station (operated by another TOC!) would sort it out for them and print their tickets. No endorsement or anything like that but they were more than happy to show the purchase on their Internet banking screen.
I was quite happy with what they said but annoyed they'd been put in a position where their word would have to be taken over a matter that could go badly for them so I rang the station supervisor to see what was what. They confirmed precisely what I had been been told and that they had been stood next to the customers when they used the TVM and seemed surprised when I said that the other booking office wouldn't be able to sort out their tickets and that they should have given them some sort of endorsement because not all inspectors will take a passenger's word.
I wrote them out a full endorsement with my details on it explaining the issue and requesting that they be treated as holding the tickets they had paid for and sent them on their way with my apologies.
I think a lot of problems, as I said above, come from staff who are willing to use their common sense and discretion not appreciating that that isn't necessarily carried forward by others.