In Safari, go to a webpage where the Reader is available, and activate the Reader with command-shift-r. Move to the end, then vo-right to loop around to the top (you must have cursor wrapping enabled in VoiceOver's preferences to do this). As you move now, some elements will be spoken where sounds might normally play, depending on your settings (specifically links). More importantly, doing a vo-a to start reading will freeze Safari completely, and just moving past text will result in VoiceOver saying just "text" instead of reading the text on the page. Even if you don't use cursor wrapping, if you activate the Reader before a page is done loading, when it loads, this bug can be triggered.
Bug First Encountered
How often the bug occurs
Command-tab away from, then back to, Safari, and that should fix things.