When trying to switch my solution from using Ankhsvn to VisualHG (we've just migrated from SVN to Mercurial) I kept getting the following message:
"The active solution or project is controlled by a different source control plug-in than the one you have selected. If you change the source control plugin, the active solution or project will be closed. Do you wish to continue?"
The solution would then close and when I re-opened it it would still be linked to Ankhsvn.
I tried deleting the source folder and grabbing a fresh clone out of Mercurial, but the problem kept coming back.