When you're sharing your entire screen, and the screen that you're sharing is visible then it's like two mirrors reflecting each other. It creates an infinite reflection effect that looks like this:


This happens with any software that lets you see the screenshare as you're screensharing. Don't worry - it's not a bug and nothing's broken (: When you switch away from the crowdcast, your audience will just see your presentation or webpage that's being shared. 

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