Caveats

• Mac OS: When pressing M-<space> nothing happens
  • Press C-h k then M-<space>. If emacs does nothing, the OS does not pass the keypress events to Emacs.
  • In the System Control Panel of Mac OS, check the shortcuts in the "Keyboard" preference pane - they should not be set to ⌘ Space.
  • In the System Control Panel, in the language and text preference pane, activate an alternative input source, change the shortcuts for switching input source then disable the alternative source again.