The standard for losing a trademark essentially boils down to "no reasonable person would expect the public to know it's a trademark, so any infringement can be assumed to be accidental". So things like dry ice, heroin, escalator, gasoline, trampoline, flip phone, and teleprompter. Common words where the fact that they were once trademarks is obscure trivia. The more commonly cited examples of genericized trademarks like Kleenex or Band-Aid are not actually genericized, that's a myth, they're in no danger of being genericized because people know they're trademarks.