I've been trying to work out why 'bathful of bikes' (http://FantasticContraption.com/?designId=169766 - I'm not insane, I was trying to solve the level under pro-wheel league rules!) works on some machines and not others.
Results so far are that it definitely works on 2 Apple Macs, but fails on 2 PCs (plus I have 2 more reports of fails on the forum).
I think this is something different to the known 'odd processors' problem, as it fails on the Macs when running PC emulation using Parallels.
If you want to test it for yourself, failure can be quickly recognised by the bike with a yellow wheel rearing up out of it's place in the bath, while success is slow, the last wheel reaches the goal after about 2 minutes 45 seconds.