First, of course, when I didn't know any better, I fed various combination of CC to some worthless units, like my +47 Log Choper (hey, he was a very good healing captain for the early game stages). Feeding a few RCV (damn the +3 traps) to some characters like Leg Mihawk was another bad idea (so now my Leg Mihawk is +100 ATK, +98 HP, +2 RCV... grrr).
Later mistakes where much more costly, like my plan of +200 all 2x HP / 2x ATK captains. Fortunately Shiki and Doffy are still pretty good, but well, +200 Raid Garp is not a particularly useful investment, I stopped with Vista at +23. Other than that, I wasted some candy +1 some characters, I thought I would mark some units I liked that way (so I have +1 Pell and Thatch and a bunch of others...).
How much candy did you waste and how?