I have seen some captains showing the brand new FA
's how to turn the airplane in flight with HDG SEL knob but I wouldnt refer to it as "taking the controls".
In my previous company we had male and female flight attendants who actually were holding different degrees of pilot licences,I personally have offered some of them, hands on training of FMC loading,or SID set up or walkaround the airplane(all on the ground) but occupying the pilots seat in flight is a different story which personally I find it not acceptable.
A few years ago I had a TCAS RA
incident while in cruise over Hungary Austria border.The MD
80(or one of the variants) flying at FL
370 with a call sigh indicating that its a Ferry flight(I dont want to say the name of the company and the flight number here) suddenly vacated the FL
with an awkward descent (definetely not a pilot maneuver) causing a RA
to be triggered,we descended and they recovered back up,but we clearly saw the company name and the aircraft type on the fuselage.After the incident from the developing sequence of it and regarding the fact that it was a ferry flight,we thought it may be someone(a non pilot) was manupulating the controls and caused this to happen.Ofcourse we dont have any proof of anykind that a nonpilot was on the controls of the other airplane but it looked like it.
So it is possible that a FA
or a non pilot can accidently touch something that he/she doesnt supposed to touch therefore it is very dangerous to allow anybody on the pilot seat in flight.