The problem with "someone on the platform told me..." is that, whilst a lot of the time it is just an excuse/attempt to sound credible, it is the case that a lot of duff information is given out. All railway staff should be able to answer straightforward questions (like "is this ticket valid on a different train") and more importantly be able to say when they don't know the answer (or look it up).
The other day I was on the 0741 off Paddington and the Guard pointed out to an Australian woman, who had just arrived into Heathrow, that her (Off-Peak) ticket wasn't valid. She pointed out that she had been sold the ticket and been told by the ticket seller as well as someone (don't know who, presume information stand) at Reading to get the 0741. I really don't think she was "trying it on" or indeed would have had the first idea about the validity of the ticket. Not really sure what I think should be done in these circumstances, just illustrates that there are some genuine cases of people getting misleading or just wrong information.
I second mumrar's post; there is a wide-ranging tendency for people to compare Anytime and Off-Peak rail fares with the bargain-basement heavily-restricted low cost fares. I do wish they wouldn't.
There also seems to be a wide-ranging tendency for people to only consider bargain-basement Advance fares when many AP fares are in fact only slightly cheaper than walk-on fares.
XC in particular appear to have an inexhaustible supply of AP tickets priced slightly cheaper than the equivalent walk-on fare. For example, Reading to Manchester on Friday, all evening fares are £52.50 AP or £65.60 Off-Peak.
My view on this (as we've had the discussion many a time before), is that it should be possible to excess an AP
before travel time to the equivalent walk-on ticket without penalty or with a small admin fee if after the booked travel time. Travelling on the wrong train should be excess with a hefty admin fee, or sold a new Off-Peak ticket. This would have the effect of making the penalty for missing your train proportionate to the fare originally paid, whilst still discouraging people from trying it on.
Apologies for rant! And I do rather like mumrar's original tale - if you're going to make up an excuse, at least spend half a second thinking it up first!