I recently found myself in a situation in our TFS Source Control where I wanted to convert a main folder into a branch. However I was presented with an error message telling me that I couldn’t do this as a branch already existed below this folder. It turns out that a branch had previously been created incorrectly and then subsequently deleted. However, if you delete a branch its relationship remains and prevents you from branching its current parent.
I found the following blog post provided me with the answer. But more usefully it showed me the ‘show deleted items’ checkbox that gives you a much better historical view IMO of the Source Control structure. Maybe this should be the default view with the option to turn it off rather than on?