My '99 E430 has done that a time or two, but not enough to cause me to think about it. I thought it was operator error.... foot not on the brake (does it have to be?), or perhaps I just did not turn the key enough. Each time if I turn it back off and try again it fires.
I think the starter is these cars is engaged only indirectly by the switch... Electronics control if and how long the starter actually cranks, and cut off cranking automatically when the car starts. Note if you just "blip" the key, the starter will crank away long enough to start the engine. If you hold the key in crank position, it will still stop cranking when the engine fires.
OK, OK, so now that I typed all that, I have no idea how it helps...
I am interested in any fix you find though, perhaps mine is not op error chalked up to senility after all.