I had a fully checked in project I wanted to apply a label to. I accidentally clicked and created a branch instead. I deleted the branch, and then correctly created the label.
I then made changes to a file. I tried to save the file (in Visual Studio), and received the error (but the file was saved). The file does not show that it is checked out, and the Branch Explorer does not show that there are any outstanding changes.
I brought up the Plastic SCM program, and the Branch Explorer there DOES show that there are changes to be checked in. If I try to check them in, I get the same error.
I haven't figured out a work around yet. I can't move forward without checking in changes.