I have seen =tons= of obfuscation (non-minecraft). Back in the late 90s it used to be popular, unfortunately.
Most of the stuff is like naming every method a or b, and using the fact they are overloaded, given one-letter-name or a reserved keyword like 'if' to classnames (or packages) was popular, too. Pretty much constant pool modifications w/o too much byte-code-editing.
Overall cheap and unnecessary and has not stopped anyone.
It's still pretty popular. Most large smartphone applications are obfuscated to some degree. At least for Android, because it's bytecode for a VM, it's still trivial to disassemble and understand what is happening at a high level.