Lots of good examples here, but from a purely hobby perspective, I just keep going back to Brien Taylor and Todd Van Poppel, whose card prices rose to then-astronomical levels based on their promise, but who never had their promise materialize, for a variety of reasons.
As a young kid, we chased rookie cards from Bob Horner and Willie Wilson for similar reasons, with similar results.
Also, John is as nice a man as there is in the hobby, and if I had a dollar for each time I worded something awkwardly, I could quit my day job.
-Al