On one incident there was a deadheading FA sitting by the overwing exit on a 727, an aircraft she was not familiar with.
When the FE started the APU it torched and some flames came up by the window she was sitting by.
She immediately screamed for everyone to evacuate, jumped up, ran across the aisle to the other side's overwing exit and opened it. Then as the confused and now frightened passengers started yelling and standing up, the assigned Cabin Crew managed to tackle the out of control deadheading FA and informed the Captain to stop the aircraft as passengers were trying to leave the cabin.
As it turned out there were no serious injuries as no passenger was actually able to leave the aircraft as it was taxing.
In the case that is the subject of this thread, tell the cockpit crew first, if they are not paying attention to you, whack them up the side of the head if you have to, but tell them before one starts to evacuate passengers via the slides.