So I was testing Silver's game and noticed that I kept falling through the floor for some reason. So I decided to make a quick platform example and see if I could replicate the problem:
<img src="http://farm9.staticflickr.com/8328/8090127806_9d8d5d60c7_z.jpg" border="0" />
^^^ Here I'm just jumping around.
<img src="http://farm9.staticflickr.com/8044/8090126995_0c7d4f7f8b_z.jpg" border="0" />
^^^ Here I fall through the platform.
All the default platform behavior settings are on, with the exception of the "Allow Down Button" which I have unchecked in the Platform Behavior. The platform is a solid.
The fall through doesn't happen immediately. You have to jump around and double jump around constantly for a few minutes, and eventually you'll fall through.
Here is a .cap so you can check it out: rapidshare.com/files/733064018/PlatformTry.cap
P.S. Hit spacebar to jump.