user-7535d

Works for me in pre5

If you have Razer Synapse installed for a Razer mouse and/or Keyboard,it will add a invisible _JAVA_OPTIONS variable to your environment variables. (It's a bug in the Razer Synapse.)

This was the problem for me.
If you have that software either uninstall it or add _JAVA_OPTIONS with the -Xmx flag you want.

Source: http://www.minecraftforum.net/forums/support/unmodified-minecraft-client/2183431-_java_options-picked-up-without-existing?comment=2

Better explanation: Jumping over a carpet block and having your lower body pass through the air part of the carpet block causes the sound of landing on wool.

Still a concern in 14w21b.

It's easy to replicate just place down one carpet and jump over it a couple of times.

Added a 14w18a screenshot of the bug