Sure. In just about every language I can think of, "42--" will cause an error. This is usually because you're trying to alter the value of a constant. As far as I know, FORTRAN is the only language which allows you to perform such a moronic action. And I don't remember whether FORTRAN even had an autodecrement operator. On the whole, you might have been better off with something like "$i = 42; $i--;".
And would any programmers out there like to explain to wirthling the magnificent what 42-- means?
Now, as to the meaning of "42 - -", which is what you really typed in the title of your comic, I've no clue. You appear to be trying to subtract a subtraction sign, or maybe the standard input stream, from a constant.
But hey, I don't know what more we should expect from a guy who can't even spell the name of his own web site correctly.
Get it right!