Hi Amy,
Doodlebuckets:
https://www.doodlebuckets.com is the site. I added an internal page called "Baby Links" and linked it to a file called links.htm Sometimes when I click on it, I get a URL of http://www.doodlebuckets.com/fuzzi_bunz/links.htm&zenid%28blah blah blah) and then I get an error page.I remeber I have had exactly the same problem. For some reason I thought it was related to the SEO url's contribution I added to the site, and I think I fixed it accordingly. Unfortunately the drive I saved the files to has crashed, but I think I can still trace it back.
The fact that it does not happen all the time is related to the fact that the zenid only is attached to the url at the first page request and when cookies are disabled. So if it is the first link the visitor clicks on, or the visitor does not accept cookies the error will occur.
Search engines should not be affected as long as they are listen in your spiders file (so they do not get a session id).
I will dig in my memory, and also try to trace the fixed file, the solution I found might work for you too :-)
I do wonder however, since this problem seems unrelated to the SEO module (or did you install the SEO url's mod, but disabled it?) it seems to be a Zen Cart bug? :unsure: