Don Frye was 2-0 against Gary Goodridge coming into their 3rd fight, and took a nasty head kick that laid him out cold.
In their current conditions, sadly I'd pick Barnett against CC in a 4th fight (to be totally fair, their first fight kind of ended up in a fluke injury)
Hughes vs Hallman has been a popular choice for the last 10 years.
And although I don't think Big Nog could beat Mir right now, he did look like a zombie in their first fight
(I remember the live PBP thread, Nog was a HUGE favorite and everyone was like "wut, what's with Nog he looks weird/stiff", but now "newer fans" act like that it never happened and just a bunch of "PRIDE nuthuggers" thought that)
, and, even it's his own fault there, in the second fight he had won, Mir was badly rocked and had Nog thrown a couple more punches the fight would have been stopped.
But he let his stupid pride have the better of him and tried for a sub... one of the best come-backs in MMA's history.