We have some unused collections and this seemed harmless, until I started accumulating problems.
The first issue happened with BPA. It refuses starting a scan, if it sees off-line collections. Not a big deal, as I put them online just for this.
I encountered a more serious issue with Lab Management configuration, so much that I opened a case with Microsoft Support.
I tried to change the Virtual Machine Manager server used by TFS, and I received an unpleasant HostShutdownException error.
TfsConfig.exe lab /settings /scvmmServerName:vmm-tfs.example.com Logging sent to file C:\ProgramData\Microsoft\Team Foundation\Server Configuration\Logs\CFG_SET_URL_1234_100001.log Microsoft (R) TfsConfig - Team Foundation Server Configuration Tool Copyright (c) Microsoft Corporation. All rights reserved. Command: lab Microsoft (R) TfsConfig - Team Foundation Server Configuration Tool Copyright (c) Microsoft Corporation. All rights reserved. Do you want to overwrite the existing settings with these new values ? (Yes/No) Y Exception of type 'Microsoft.TeamFoundation.Framework.Server.HostShutdownException' was thrown.
As we discovered, and you probably guessed, it was caused by the off-line collections. Bringing those online solved the issue.
Now you know.