Yordan Alvarez to the Astros
Yeah, this one definitely hurts a bit in hindsight, but was somewhat understandable at the time. The Dodgers needed help in their bullpen back in 2016 at the trade deadline. They had spent big money to sign Yordan Alvarez out of Cuba and while they still liked him, he wasn't mashing the ball like he is in the big leagues right now. So, they made the decision to send him to the Astros in exchange for reliever Josh Fields.
Fields did well for the Dodgers across 2.5 seasons with ERAs under 3.00 each year, totaling 124 appearances before his big league career came to an end in 2018. The Dodgers got exactly what they wanted out of him. Unfortunately, it cost them a guy that turned into one of the best power hitters on planet Earth right now.
Rick Sutcliffe to the Indians
Whenever you trade a guy that goes on to win a Cy Young, it never feels great, and that's what happened when the Dodgers sent Rick Sutcliffe to the Indians back in 1981 for a package of prospects. While one can be certain that the Dodgers liked the prospects they got at the time, the return for a guy that had won Rookie of the Year feels like it needed to be better than what the Dodgers got.
Sutcliffe would go on to finish out an 18-year career that included three top-five Cy Young finishes, including a win in 1984, and 31.2 rWAR throughout his career. As for Jorge Orta, Jack Fimple, and Larry White (the Dodgers' return in that ill-fated trade), none of them meaningfully contributed to the Dodgers at all.
Carlos Santana to the Indians
Back in 2008, Carlos Santana was considered by many to be one of the Dodgers' best minor-league prospects since catchers that can mash are tough to come by. However, that didn't stop LA from moving him to the Indians in deal to acquire third baseman Casey Blake near the trade deadline.
In fairness to Blake, he played well for the Dodgers after the trade, posting a 4.6 rWAR season in 2009. The Dodgers wanted production and they got it out of him. Santana, though, has gone on to have a very nice 14-year career (and counting), which includes six 20+ homer seasons and a 33.6 rWAR career.
Paul Konerko to the Reds
This move comes from the time when Tommy Lasorda was in the general manager's chair back in 1998. LA needed some help in the bullpen, so they traded then-rookie Paul Konerko and swing man Dennys Reyes to the Reds (why is it always the Reds?) for reliever Jeff Shaw.
Konerko's time in Cincinnati was short-lived, but he would join the White Sox in 1999 and play for 16 seasons. He made six All-Star Games and was one of Chicago's most consistent offensive threats until he started to tail off the last couple of years. It sure would have been nice to see what he could've done in a Dodgers uniform even though Shaw did pitch well for LA across four seasons.