Guys: KISS. You are sitting on the ground, warm & cosy. You receive a clearance to leap into the unknown with confusion in your mind. Your warm cosy feeling is disturbed. What would you do? Take a leap of faith that it'll be alright on the night and you'll sort it out with ATC later; or pause, consider and ask again? Then you can leap off and still feel warm & cosy knowing exactly what you re going to do. Let's not use too much energy trying to find excuses for why FL 200 was mistaken for 2000' once airborne. That is too late to be confused.