I know you were looking for one trade, however, as I scoured the history of the Blue Jays, I actually picked out three that really let me down.
The first of these was in 1979 when the Jays traded Alan Ashby, a solid switch hitting catcher for pitcher Mark Lemongello. I remember being aghast at this trade. I was a real fan of Ashby, but at the time, Lemongello was praised as the starter we needed, however, he was nothing less than brutal. We may never have needed Buck as a back up catcher if we had kept the duo of Whitt and Ashby.
The second trade, essentially Michael Young for Estaban Loiza, We suffered many years at shortstop and Young would have been perfect, instead, we ended up with a mediocre starter, who either underperformed or was just the way he was.
Finally, the Jays made a great trade to rid themselves of Vernon Wells salary and received a mediocre outfielder in Juan Rivera and the potentially laden Mike Napoli. Then, just a few days later, they offload Napoli for Frank Francisco, who was going downhill fast and accelerated the decline once he joined the Jays. I heard an interview with Napoli on the FAN590 just after the initial trade and the sounded genuinely excited to be joining the Jays. If only we had held on to him for the year and even resigned him, we would have had an even more powerful lineup.