The favorite I was not involved in, was when they spent many months (at least 3, maaaaybe more than 6?) working on a thing, with verbal promises on the other side, but then the owner of the business that kept promising to sign a contract once we got to a certain point died and at that point the whole thing got quietly killed.
Again, not my project; the ones I was involved in (different product,) at least half of which did materialize but most of them never made up the volume for the pain of the work, either on the dev side or the business side [0].
[0] - My favorite was when we did an integration with Tesla. they decided to use a VIN as the primary key for their salesforce integration. So, if a customer got an inspection on a trade-in, waited, then decided to try again... oops!
We were supposed to make it all up on being able to hard-sell the user on addons and multi-pack buys.