When the xDB.Tracker identifier goes missing.
If you are on Sitecore 9.0 to 9.1.1, and you have no patches, then I am afraid that you are losing data. Here is how you can find out if you are losing data.
Our little analytics DTO project is getting all grown up. Now called xDB Peek. An easy to use, styled tool that lets you see everything you need to know about a Sitecore visitor to your web site.
When you start creating xConnect facets, one of the first things you want to do is show them. So you open Google and start your search. Oh joy, I found a 30 step process to do it in Speak. Some time (most times) you want to do it in less than 3 days :)
In the Sitecore 9.x shard databases, the identifiers are hexed. Making it a little harder to query for them. Lets right some sql to do it.
I have to know what Sitecore is seeing about my user/session. This includes everything from goals, events, profiles, patterns, pages, geo, marketing automation and now xConnect facets.
In 8.2 it was easy to reset a facet in code using the .Reset() extension on the facet. But in 9.x they removed this making it difficult to clear a facet. Lets fix that.