Ok I admit it. I took the easy way out. I was setting up a test environment of two instances using the standalone graphical installer and once done, I switched their roles. Everything worked except for one thing; the Experience Profile dashboard.

An Administrator's error caused the SQL account for Sitecore's collection database to be denied login. Normally you'd expect the related services to stop functioning, but in this case the system's various resources spiraled out of control.

It's taken years of managing Sitecore sites for this one to come across my desk, but apparently there's no protection for the UserAgent value being longer than the maximum allowed length when saving the User's session to the xDB. Though it's fixed since version 10, there's no patch for earlier instances, so we're going to have to add our own pipeline processor to cut the offending UserAgent values short.

While managing an instance of Sitecore I saw a bot got past the excludedUserAgents configuration, which means its traffic is recorded and we don't want that. Flushing the data out is simple with the Analytics Database Manager, but there are cases where there's transient data queued for processing in the GenericProcessingPool table, and it can be a problem.