>>> AN APP IS PLAYING UP

If an app behaves oddly, quit (or force quit — see page 19) and reopen it, restart your Mac, and check for updates (in the Mac App Store or from the app’s menus). No joy? The app’s preferences may be corrupted. Quit the app, switch to Finder and go to ~/ Library/Preferences (see ‘Where’s my Library folder?’, page 20) and drag anything obviously named after that app to the Trash. With luck the app will now work, but you may have to set its options to your liking.