What I can think of is that the platform is short (only under the layout). If you take sonic past the boundaries of the screen, it will fall in the void because there's no more platform there.
Also, auto-mirror doesn't work for me. Sonic still face right, even when I'm pressing the left key.
And the simulate jump don't work in rows. I mean, I'll press the key up once and I have to press "shift" before the "up" key acts again.
Pressing shift works continuously.
(the test is done after having "reworked" the events:
Start of layout
+ Mask set to invisible
Keyb Up arrow is pressed
+ Sonic simulate jump
I don't know if that is the intended behavior for the simulate action.
C2 r54; Windows XP SP3; FireFox 6.0.1; ATI Radeon HD 4870