I accidentally set a sharing intent to always copy to clipboard. I know that usually to unbind these defaults, you go Settings > Apps > All > [bound app] and hit the 'Clear defaults' button from there, but I've had a look at all the system- or text-related things in there and I can't find anything with such a default set.
In my googling, I found this thread on XDA Developers that describes exactly this problem, but it has no responses.
If it matters, the action in question is the one the Github app uses to share issues.