I dunno about UA, but other carriers give you an error message if you try to check in online when you've been selected for harassment.

And now, here's how to avoid getting harassed at the checkpoint. If you cannot check in online, as is the case in this scenario, buy a fully-refundable ticket on another carrier that shares the same concourse. (at DEN, all the concourses are connected, so no matter) Preferrably, buy this ticket on a carrier with whom you have status and make sure it's round trip and not one way, as all terrorists are too stupid to realize that one way ticketing is often more expensive than round trip. Check in, use your new BP at checkpoint security, refund the ticket over the phone. Present your original SSSS harassment ticket at your original flight's gate. At worst, you'll get the harassment you would have received at the checkpoint. More likely, you'll just get boarded.

Admiral Loy promised to get rid of stupid rules a long time ago. SSSS is just another useless one that is so easy to bypass that the people who insist it remains should be criminally prosecuted for negligence.

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"Give me Liberty or give me Death." - Patrick Henry