Actualy it makes perfect sense. If the site had been accessed when the db was unavailable, the proxy would have cached the resulting error page. What got stred in the cache would have been served upagain in later access to the site. However, other browsers that do not use the proxy would have seen what you expected. Eventualy what is in the cache expired and the proxy server re-grabbed what was available.
The reverse is also true, if the first time the proxy hit it and it worked, then the proxy would have cached that. Then if you hit it w/o the proxy and it had gone down, it would give the appearance that it was working in one browser and not the other