Most recent for me was Jon'Vea Johnson. He was getting great separation his rookie camp, but wasn't making the catches in the preseason games. I thought once the coaches worked with him on that, he would be a great find. However, he didn't make any steps forward last year and was cut in March.
I usually pull for our UDFAs, but I full-on adopted Johnson and actually promoted him as a pet cat and ended up getting bit for it. I don't know if he's my all-time overrated, but I'm usually more cautious about my expectations for players.