Happened to me on an ANA flight from Rome to Tokyo...luckily C was virtually empty and I swear those sweet Japanese FAs kept a lav free for me!
Happened to a buddy on China Airlines from TPE to NRT. He was in there the whole flight. I said he should apply to the airline to get a refund of the difference between C and Y. It was most probably the airline's prwn cocktail that did it....