I think, it would be nice to add to the FAQ, Manual or "Frequently asked questions topic" information about 2 important problems.
I've tried to define controls for 2 players on 1 keyboard (it's very necessary feature for fighting or football game, or even TDS with split-screen), and found out that
2) Most of keyboards have troubles with detecting arrow-keys and numpad keys, pressed at the same time.
(try to press up-arrow +left or rigt arrow+ num0, no effect).
Maybe there is another unobvious problems :) Does anyone know about it?
Regarding your second point: my current project makes extensive use of 8-directional shooting using the four arrow keys (and/or numpad) in every conceivable configuration and I haven't noticed an issue yet.
Fmistakes - Could be your keyboard just doesn't support Arrows + Numpad toches at the same time? : P
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Maybe so, but some of my friends have the same problem, and it's useful information if your games are commercial and users with the same keyboards will scream "OMG WTF I'VE BOUGHT THIS GAME, BUT SOMETIMES MY KEYBOARD DOESN'T DETECT THE KEYS " :D