Mine also was intentional. Verichrome Pan @ ISO 1600. It was for a fashion photog who liked to push the envelope. I found a recipe for ISO 800 in an ancient Amphoto loose leaf book and added about 25% to the time. He won a contest with the print.
i was photographing a lawyer for a magazine article, and realized after i left and souped 1 of the 8 sheets of film ("test sheet") i exposed and it was clear --- that i had underexposed my film 5 stops. he was gone by the time i realized my blunder (530pm), and a print was due at 8am.
i processed 1 sheet at a time until i got something, and then printed on some strange agfa grade 5 paper i had ...
Maco UP400+ (I don't know all the rebrandings, so I'll just call it what it says on the box), pushed to 1600 - I actually got a lot of great shots on that one roll. Also, APX 400 pulled to 100, excellent tonality, good shots there too.
I've had one or two accidents when I forgot the cartridge for my 400iso film was DX coded for 100iso though...