In the few games that I tried to create ( yes, many left in development because of time and mood ) I rarely used key combinations. The ones I am talking about right now refers to arrows and space keys. Some combination of three of these give a lot of troubles and to a lot of people seems they are not working.
I found out that it’s not a Flash issue but more a hardware issue on some keyboards and the solution to that is not using arrow keys as your controls. WASD seems to be the best choice.