Chaim and Shprintza got engaged. They, their entire families, and their rabbis were all totally happy about the engagement. They didn't hide any information from one another. Chaim and Shprintza both know both his and her detailed halachic status.
A few days later, Chaim and Shprintza (and their families) still desperately want to marry each other, but halacha now requires them to break their engagement. There were no mistakes at the time of the engagement. Chaim is not a Cohen.