If a negative number is used for the <S> parameter (starting position) mirc spits an error:
* /bread: error accessing <filename>
Example: //bread versions.txt
-2 2 &a | echo -s $bvar(&a,1-)
If <S> is a non-numeric (such as "a" fex), mirc treats it as 0, so it starts reading from the beginning of the file. I believe this is what it should do with negative numbers as well (or at least give a proper error message, like 'invalid parameters').