Multiwire Branch tripping off

My client informed me that, when visiting the home(days before the inspection), his father (an electrician) flipped on a 20amp breaker in the panel that was labeled “Do not turn on”. When he did, it tripped the power to it and the adjacent breaker.
When I inspected the panel, it appeared to be a multiwire branch circuit powering some outlets in the kitchen. I wrote the issue up and recommend evaluation and correction by a licensed electrical contractor.
Obviously the person wiring this was aware of the situation and should have corrected it, however, my question is…What circumstances could cause this?

Breaker on red leg and breaker on Black leg both connected to same out let. This involved a direct short and caused both breakers to dump is one explanation I am sure we could come up with others too.
Sounds like some one was trying to make a split plug and forgot to remove the jumper .

.ADDED LATER> If that is the case then it should have been on a two pole breaker

As Roy stated something was mis-wired. Simple way to check is to see if there is voltage (should be 120 volts) on the conductor connected to the breaker that’s off.

Thanks, I’ll keep that in mind in the future

That would be my first guess…

Did he really ask that?

Must be a new GEICO " When Pigs Fly" commercial ?!