I had something similar, but I was able to reproduce it....
OK... it just happened, but normally users should not be doing this.. allthough you never know...
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1st I look at a category:
http://www.mysite.com/forum/gallery/browseimages.php?c=18&userid=
then.. just for testing.. i change c=18 to c=1000
http://www.mysite.com/forum/gallery/browseimages.php?c=1000&userid
I get the following error message:
No Category specified. If you followed a valid link, please notify the administrator
that is ok.. because the category does not exist...
BUT:
the word "administrator" is linked... when i click on it, my browser wants to go to the following url:
http://www.mysite.com/forum/gallery/admin-email@adress.net
admin-email@adress.net being the contact email I entered in the board!!!
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so I guess if there is a wrong link somewhere or a place you users are not allowed to go and the system tells you should contact the admin if you think there is an error... in that case the administrator link appears.
in your case
if a search engine is just indexing a page.. it could be that it would try to follow that link that it is not allowed to.. and you will have it in the logfiles...
q.e.d.
Luciano