I've pushed my Casio quite hard and haven't experienced any stuck notes. Check the bottom of the board to see if it's a refurb. My first WK-3500 was a refurb, and it was DOA. The power turned on, but nothing after that. Just a blank blue screen.
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.