Dissembling my Toa Mahri. I had all but Hewkii, and now all I have are parts. Since then I've managed to reassemble Jaller, Nuparu, and Kongu (mostly).
Although, honestly, Hahli probably wouldn't be around anymore anyway, because of those stupid lime-green pieces.
Oh, and trading Ehlek for Transformers 1 Arcee.
I hate that I did that.