I can't use the scroll wheel in Gedit. The scroll up or down events now have the same effect as right clicking.
I'm not sure what happened. I never messed with any settings. This happened while Gedit was open. Restarting Gedit has no effect, rebooting has no effect, plugging in a different mouse has no effect, and it's not a problem in any other application.
The command
xmodmap -e 'pointer = default'
will restore different actions of mouse to defaults.