flixel is a free Actionscript (Flash) library that I distilled from a variety of Flash games that I've worked on over the last couple years, including Gravity Hook, Fathom and Canabalt. It's primary function is to provide some useful base classes that you can extend to make your own game objects.
There is a danger that at the first time a call of the function step() may be skipped (when you set high framerate, for example FlxG.flashFramerate = 90) and _state.draw() will fail.
https://github.com/krix/flixel/commit/7fbc822359dfdfd6d74d9b9eaf86e0699c1cb0da
More explanation: http://forums.flixel.org/index.php/topic,5072.0.html