There are several situations that might cause passengers concern prior to takeoff, and flames shooting out of an engine is certainly one of them.
Passengers on an Utair flight from Moscow to Makhachkala, a town on the Caspian Sea in Russia, panicked when they spotted flames coming from under one of the engines of the Boeing 737 in which they were seated. Those in the emergency exit row opened the door and three people climbed out of the plane and onto the wing.