I've been stuck on the wrong train three times and nearly done it a fourth time.
The First Occasion was during the Thameslink blockades in London, I was racing back home to see a football match, on TV, after work. I got a 1900 (I think) MML service out of St Pancras, knowing I could change trains at Luton and save myself 15 minutes on the journey home. Arriving at Luton I knew there was a 1924 service to Bedford calling all stations, so I hot footed it from Platform 5 to 3 and jumped on the waiting train just before the doors shut. It was only as the train thundered through Leagrave that I remembered there was an extra train in the evening at 1922, first stop Bedford! I missed half the match....
The second occasion was at Birmingham Moor Street, I had already been delayed getting there and, having never been there before, I somehow made it to the wrong platform, as a train was in the platform, and went the wrong way, Fortunately the mistake didn't result in any overall delay.
On the third occasion I was at Warrington Bank Quay, waiting to head to Wolverhampton for an onward connection. The departure boards showed my train, the announcements made were for my train, but the train that arrived wasn't my train. This only became apparent when an announcement was made after departure. As I was ultimately headed to Didcot I was not delayed by the diversion to Euston, and the staff were kind enough to let me travel onward with no penalty.
The other time, when I nearly went on the wrong train, was at Norwich. I had arrived in from Ely and went in search of a train to Cromer. I found a screen, looked up to see "Cromer" and the platform, ran to it and got on. A few moments later an announcement was made, it wasn't going to Cromer. So I went back to the departure screen and had another look. Turns out someone thought having an arrivals screen on the platform side of the ticket barrier was a good idea!.....