I've run through installing the recent 2015.1 update on our test server, and it got to the end but with a couple of disturbing caveats.
TF401003: A backup copy of your existing web.config was saved ... yada yada.
TF401005: Some customisations were not successfully transfer from your previous web.config file. For more details, see the configuration log.
As far as I am aware, we had *not* made any customisations to the web.config file.
And looking through the log file produced by the update, I do not see any mention of "web.config" or "TF401005". What exactly am I supposed to be looking for?
I ran a DIFF on the two files and can see
1. a module called SignalRRoutingModule was removed.
2. a reference to the System.Runtime, 4.0.0.0 was added
3. the <allow> entries in the authorization section were typeset differently but otherwise unchanged.
4. a compiler specification was changed, along with a reference to using Roslyn
5. two new appSettings were added (aspnet:UseTaskFriendlySynchronizationContext and aspnet:MaxConcurrentCompilations)
6. a key "staticContentVersion" changed value from "12" to "Dev14.M89-Part7"
7. a key (owin:AutomaticAppStartup = false) was added
8. a slab of <system.diagnostics> was removed
I don't recognise any of those as being something I consciously did during the initial TFS installation/migration processes.