In the Dash-8, there have had a couple of instances of cellphones being used in the cabin's rearmost seats creating interference with the baggage compartment smoke detection system, causing the smoke warning light to come on in the cockpit. This on one particular occaision occured in flight; with no other way to verify whether there was actually a fire or not without opening the aft bulkhead door and potentially endangering everyone on board, the crew donned the full oxygen mask / smoke goggles regalia, declared an emergency and diverted to a nearby airport.
On the ground they discovered 7 people in the back rows with cellphones turned on, several of whom had apparently tried to sneak out a call while the flight attendant was busy in the front, pretty much simultaneously. There were some choice words said, and stiff fines imposed, from what I understand.