Steps to reproduce this bug: 1. Open Gmail in Safari's full screen mode 2. Download a PDF attachment 3. Preview will open automatically downloaded file and the dock will disappear The issue seems to be related with moving between spaces while opening an app. Dock will not re-appear until we hide / unhide it in preferences or until we swipe to another space and swipe back. This happens regardless of the position of the dock. FYI: my rMBP came with ML pre-installed. Edit: this issue obviously doesn't happen if we disable Safari from opening safe files automatically.