Isolated Missing Visitors

I recently noticed a few isolated visits which are not tracked in Piwik 1.7.1 despite:

[li] The vast majority of visits are tracking fine.
[/li][li] There is no Do Not Track plugin or other privacy enabled.
[/li][li] The IP address is not blocked.
[/li][li] piwik.php is being successfully sent for each page with what looks like good data.
[/li][li] I resent the exact same query string using “curl” from one of my own machines and that did show up in the visitor log.

The only thing I can’t see/reproduce are the cookies. But I didn’t think there were any cookies whose presence or absence should cause an otherwise good piwik.php call to be ignored.

So what else could cause a visit to not show up in Piwik?

The “exclude cookie” could cause the request not to be tracked.

Otherwise you can try debugging:

Thanks for idea about about cookies, but the missing hits are from unknown (public) website visitors, not Piwik users. Is there some mechanism by which such a person could set such a cookie?

With regard to debugging, am I reading correctly that enabling that would cause debugging information to be displayed on the public pages being tracked?