Well I had a clearance the other day "Descend FL 80 to be FL90 or below by 10 miles before XXXX" - for some reason I read this back as "Descend FL70 to be FL80 or below 10 miles before XXXX" - ATC retorted "ALL WRONG - Descend etc...." It was quite busy at the time but on reflection I was almost tempted to say "Well I got the callsign right didnt I ?!"