If when flicking through presets you happen to have a key held down when selecting a new sound the note gets stuck on. its easily reset by changing the preset again but its annoying none the less.
Mine will do this too once in a while. If you press both of the octave switch keys and hold them for a second or 2 it will reset any stuck notes.
Mine does this constantly if I change a patch while playing the Sub37 Keyboard. If I play the sounds in the Sub37 from my Prophet 08 keyboard and change patches on the Sub37, they are fine.