SPUserCollection.Remove() throws misleading exception “Cannot complete this action”

[Important News!] I’ve kicked off a new project/band where I’m creating music with a heavier techno/metal sound called binarywaste.  You should check it out by clicking right here! Over the weekend we performed a User Migration of our primary SharePoint 2010 farm.  In our case, which is probably a quite common scenario, we had users from both the …