It's hard for a single player to be involved in two bad trades for the same team. Granted it's not impossible (See Samuel, Juan), but usually it's only one of the trades that's a bad one (See Alomar, Roberto). Kevin McReynolds falls into the former category of a player who was a bad pickup, and then was also a bad tradeaway.
Traded with Gene Walter (who would go on to pitch less than 17 innings for the Mets) and Adam Ging (who never made the majors), from the San Diego Padres, to the Mets in exchange for Kevin Mitchell, Stan Jefferson (who was basically a part-time player for 6 seasons in MLB) and Shawn Abner (another part-time player for 6 seasons in MLB) on Dec. 11, 1986.
Although McReynolds was actually quite a nice player during his time with the Mets (posting OPS+'s of 117, 142, 125, 121, 108 from 1987-1991), he will forever be remembered as the player for whom the Mets traded Kevin Mitchell (career OPS+ 142 in 13 MLB seasons).
The only year when this trade was a victory for the Mets was 1988 when Mitchell's OPS+ was "only" 121, while McReynolds' was 142. The Mets would go on to lose to Orel "F**in" Hershiser in the NLCS. Thereafter Mitchell would go on to be league MVP (not a great metric for establishing greatness, I know, but still he was really good) and one of the most prolific HR hitters of the late 80's/early 90's.
Thus in 1991, coming of an average season (OPS+ 108), the Mets decided to package him, Gregg Jefferies and Keith Miller (who would play part-time in parts of his 4 remaining MLB seasons) for Bret Saberhagen and Bill Pecota (who would OPS+ 69 in part-time action for the Mets in 1992).
Saberhagen was mostly hurt in 3.5 years with the Mets, although he did post an ERA+ of of 152 in 177 IP in 1994. McReynolds had an adequate year in 1992, posting an OPS+ of 115 and then was basically a non-factor thereafter, retiring in 1994 (after being traded back to the Mets for Vince Coleman, in the only trade McReynolds was involved in that wasn't a disaster for the Mets). Jefferies, however, would go on to have a fine 14 year career, with a career OPS+ of 106, and career OBP of .344. Jefferies best years came in 1993 and 1994 as a member of the St. Louis Cardinals (OPS+ of 141 and 130, respectively).
So in sum Kevin McReynolds, although he had a couple of nice years for the Mets, was a big piece of crap who screwed us not once, but twice. Thanks, Jerk!