If the calc worked properly, this would give an error, due to the division by zero. What it actually does is print HELLO WORLD. This is because when it tries to understand the program, it recognises the apostrophe as a comment and jumps to after the next newline, colon or disp. When it looks for 'Lbl 0', however, it 'cunningly' ignores any which are inside strings. Or so Casio think. It thinks that everything between the two " signs is in a string, so it only finds the second 'Lbl 0', and carries on from there...
This was fixed (again accidentally, I suspect) for 9x50s onwards.
Thanks to Tom Wheeley for reminding me of that one.