There are several problems within Vocaloid that make a voicebank unstable. You can remove these problems with sound editing software, in each case however the method will change. In the case of odd tones, background music may hide the sounds, in the case of Croakiness. Good cleaning of the samples, however, hides much of the problems with a voicebank and can often improve vocal results.
Several Vocaloids do this including, but not exclusive to; Kamui Gakupo, Prima, tonio, Kiyoteru Hiyama.
When the Vocaloid 2 voicebanks are transferred to Vocaloid 3, this "pluck" becomes much less noticable.