Hello,
What version of Kentico
CMS are you running? I wasn't able to reproduce that behavior on my 5.5.22R2 site. I'm always correctly redirected to the first page when I append incorrect value for the pager query string (different from a integer number, or some higher number).
If you have the same version, would it be possible to see that behavior on your live site (if any)?
Best regards
Ondrej Vasil