What (potential) errors does the addition of the '--' prevent?

On systems that do depend on the shebang to find the executable, a stray carriage return (as so often happens when transferring files from Windows to Unix) can break that magic. Adding -- in such cases ensures that the trailing ^M never interferes with the binary name. Then again, adding ANY switch seems to accomplishes the same...

$ cat -e cM.pl
#!/usr/bin/perl^M$
^M$
print "Hello, World!";^M$

$ ./cM.pl
: bad interpreter: No such file or directory

$ cat -e cM--.pl
#!/usr/bin/perl --^M$
^M$
print "Hello, World!";^M$

$ ./cM--.pl
Hello, World!
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    --k.