Very simple to fake problems that would convince any and all passengers, including any dead-heading pilots that there is a mechanical problem that would necessitate a return.
I will not give any examples here, however, any airline/professional jet pilot will be able to figure it out.
Also, just in case, I would request that there not be any speculation on this subject here on an open website.
(Okay one example; Have the Captain run down the aisle crying, screaming and fall to the floor sobbing saying "We are all going to die" could work.
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