Comment 6 for bug 1298668

Finally found a solution:
http://askubuntu.com/questions/598099/numpad-buttons-going-weird

dconf reset /org/gnome/desktop/input-sources/xkb-options

or

gsettings reset org.gnome.desktop.input-sources xkb-options

The problem was:
dconf read /org/gnome/desktop/input-sources/xkb-options
['keypad:future']
You can test the pain yourself with this command:
dconf write /org/gnome/desktop/input-sources/xkb-options "['keypad:future']"
And then try to use the calculator.