I recently started having an issue. I am unsure as to how long it has existed, but it must have occurred during the past week. Whenever I type Shift-c, Emacs does not interpret it as the character C, but rather the Control command. This acts only in the outline minor mode (regular outline mode is unaffected). I am unsure if this is a component of the mode, or a bug somewhere
I had the same problem with outline-minor-mode and the following line in my init.el was the culprit:
(setq outline-minor-mode-prefix "C-c C-o")
Seems like that was interpreting the first "C" as Shift-C instead of Control.