Are there any recorded (recorded, perhaps, in responsa) instances of a failed sale of chametz (e.g., the gentile refused to buy the stuff and another couldn't be found, or the rabbi forgot to do it), in modern times (i.e., in a case that a whole community, and not just an individual tavernkeeper, was relying on the sale)? What was done about it?
A certain Beis Din in Yerushalayim found out in about 1995 that they had been selling for quite a few years to someone who was really a Jew. I do not have any online sources nor do I wish to give away my sources for concerns of Lashon Hara.
It happened to my father-in-law - the community forgot to sell his Chametz one year. Since then he has always sold it himself directly to a non-Jew. But I don't think they forgot everyone's, they just misplaced his paper.