Strange behaviour on my site - when I try to create an alias of icons/* that url seems to be accepted, but whenever I try to visit the alias I get a 404 error (and not the drupal 404 error).

If I go to node/* it's fine.

If i edit the alias to start with something else, it's fine.

There is no icons directory

This is on a subdomain - could that be connected?
Or is there something I don't know about htaccess?