If you've got time to kill, there's always the nop instruction. It takes no arguments, and it doesn't do anything at all:
nop ; Do nothing
It is useful as a spacefiller during the code-generation process when writing bytecodes directly rather than using Oolong. Because the operation doesn't do anything, you can reserve space in the class file you are building with a number of nops. Later, you can go back and replace the nops with operations that actually do something. If you don't replace them, then the correctness of your program is not affected, because the nops do nothing.
Of course, this works only when you are assembling class files. Once the class has been loaded into memory, it cannot be changed.