Where to start...
I had a passenger come up to me at Man Picc after being kicked off a TPE Cleethorpes arrival asking for directions with an extremely strong Eastern European accent and fairly poor English. I couldn't get the name of the station he was saying, but it didn't sound local. He then went into his bag and came out with a ticket and a tube map which had Kings Cross and Liverpool Street circled on it. He'd come from Grimsby and hadn't understood he needed to change at Doncaster, and thought he was at London Kings Cross. Oops. The Avanti staff took pity on him and put him on the last London service with less than a minute to spare.
I remember my first trip to London with my parents, boarding at Warrington on a non stop Pendo. We headed down the train to find our seats, containing a drunk Glaswegian asleep. He woke up and asked if we were at Warrington yet as we sped through Acton Bridge...
And then the ones I've done myself:
Heading back from Blackpool after a day and evening on the adult beverages, changed at Preston onto a 185 to Manchester Oxford Road. Woke up wondering why I was still on the train at 23:40 then saw Heald Green out the window. Oops.
Got woke up by a kind fellow passenger in 1st Class on an SWR 444 the afternoon after being on night shift telling me I'd need to move forward if I were going beyond Bournemouth. This was while we were in the platform at Pokesdown. I wanted Christchurch. I didn't manage to gather my stuff in time so had an unintentional trip to Bournemouth.
I remember reading a tale on here, which admittedly may or may not have been true, of a passenger on the Caledonian Sleeper getting friendly with a gentleman in the Aberdeen lounge car before both heading back to his cabin to Err... y'know. She woke up on the East Coast of Scotland heading to Aberdeen while her belongings were in her empty cabin heading to Fort William!