Markoz wrote on Sep 16^th, 2010 at 11:11am:

I find it will do this if I try to change the setting with the mouse wheel. Just make sure you use the LMB and RMB (Left Mouse Button and Right Mouse Button) only. Hope this helps. Mark

Yes of course - don't no what I was thinking of... :) I guess it's because I don't usually decent via the CDU/VNAV but leave ATC to decide when and how much + runway assignment and heading directions..SO, all together the cockpit is pretty busy during a very short time - that is, at the same time doing both the 1 officer and the captains job! Things are then close to panic and you just sometimes tend to use your tools in the basic way, that you are use to.    
But I will be aware of it now! Promise!
However it's a little annoying that you actually can use the scroll wheel every other place - ONLY not on the speed knob!  :-/