This one is quick. You're running Sierra and you've downloaded an app from the web, but when you try to open it you get an error that says "appname.app is damaged and cannot be opened..." Sierra gatekeeper security is strict, but there is an easy way to enable non-certified apps to open.
**proceed at your own risk, apps that the system doesn't want to open could be malicious**
to re-enable the security follow the same steps but instead enter: sudo spctl --master-enable
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