I am getting
UpdatedConcurrencyException: The object SPXmlDocCacheRefreshSentinel was updated by another user. in ULS during a web application scoped feature uninstall (as part of a solution uninstall). The feature is removing WebConfigModifications; it may have something to do with that.
I'm not sure if it's actually causing a problem, but I'm wondering if anyone knows in more detail what SPXmlDocCache is? Google isn't coming up with much.