Constant spam of Item entity xxx has no Item on clientconsole

Almost every time when diggig and chopping down trees a new console message apears on the client-side: Client> [time] [Client thread/ERROR]: Item entity xxx has no Item?! where the integer value xxx increases every time.

The titlebar says 1.6pre, but I'm currently playing 1.7.1pre.This is caused by a batchfile I used to run the launcher. I like to have Minecraft sits in its own directory to be easier to be copied to USB or another system or even backed up 😉

Batchfile-content:

set appdata=%cd%
start "Minecraft 1.6pre" java -Xmx512M -jar %APPDATA%\.minecraft\minecraft.jar