I havent (touch wood) had any problems with Lipos yet, and never leave them charging when im not close by.
I did however find a flaw in Hitec chargers about 1 year ago, whereby the Pb charging mode would not actually cut off at the full voltage, the beeping would happen and the screen showed it as finished, but the ammeter on the PS still showed the current as being used.
The result was a very very hot and puffed leaky Pb battery on my desk at work the next morning.
They immediately took it seriously and changed the firmware, but I dont know if old chargers were updated..........
If this happened with a Lipo you would be really gutted, and, as a precaution, I made a relativly safe place to charge batteries by taking some metal shelving and boxing in five of the six sides with metal sheeting. nothing was stored on top of the shelves and it was in a room with a smoke detector.
I thought about using an arduino to monitor temps at the charging locations and cut off power if it got too hot. but never did.