Do you have Kaspersky Internet Security? If so, go to application management, manage applications, then find ALL Java and Minecraft entries, then double click, then in the new window, press "Exclusions" and check all of them except for "Do not inherit restrictions from the parent process (Application)". You will have to do this every reboot.
Same with carpet. Falling onto it causes this as well
fixed ❓ in 1.7.4
Duplicate of about 200 other reports. Please search next time. Fixed in 1.7.4 ❓
Confirmed on 0.6.1 iOS 6.1.3 iPod 4G unjailbroken and jailbroken. Currently have a full chest of doors! 😃
Confirmed on 1.7.2 with MacBook air.
Get an NBTEditor and make the UDUDMost a LONG instead of an INT