Most of the time this is solved by doing a "Save All", but sometimes I've noticed that I still get this error after doing a "Save All". If this happens you can solve it but doing a "Close All Documents"
This will most likely bring up another save dialog for some files which presumably didn't get saved by the "Save All". After doing the "Close All Documents" and "Save" (if asked for), you should be able to check in without a problem.
