header-footer.xml XML Invalid - Error!

[vanfruniken]

I tried to add a <script src=...></script> tag to the header field of header-footer admin.
Upon saving, GetSimpleAdmin hangs. Only deleting of header-footer.xml seems to help.
Is there any kind of syntax that I should avoid in those header and footer textboxes?
The headerfooter plugin shows great promise, but his hasn't materialized for me. What could I be doing wrong?