Can anyone tell me what the following error means? :(

"Exception in thread "main" java.lang.ClassFormatError: LVTT entry for 'ent' in class file com/sforce/ws/transport/JdkHttpTransport does not match any LVT entry"

That package that its referring to, is a precompiled one, and i dont think i can edit it, and recompile.
Any idea what could be causing this, and how i can possibly fix this.

I am using the Java 1.6

Thanks in advance.