When I press I in the Action Editor (with a bone selected in Pose Mode and no keying set selected), the Insert Keyframes menu that appears does not contain all of the normal options like Location, Rotation, Scale, LocRot, etc. Instead it only contains 3 options: All Channels, Only Selected Channels, and In Active Groups. Selecting any of these options yields no results (not even an error message) other than the file size increasing slightly.
In the screenshot below, I've adjusted the rotation and scale of a bone and attempted to add a keyframe to the current action to illustrate what I'm talking about:
I've tried changing the keying set to rotation and scale but the results were exactly the same.
As a bonus, it would be nice if I could figure out what exactly was happening that increased the file's size when I clicked on those three options so that I can undo it!
I'm fairly new to Blender, but I've been able to animate a few characters now without running into this issue before. I'm not sure what I did differently this time, but hopefully I can avoid whatever mistake I made in the future!