Yeah but they will overheat, create lots of gas, and if by chance the battery does not have a vent that gas will just build up pressure, and sooner or later the casing will fail and there will be a large release of energy that most would say the battery exploded.
Really though, 3rd party batteries are probably almost as safe as OEM. The only reason I'd say OEM is safer is it is a known quality. Buying 3rd party its very hard to judge quality. But still I wouldn't worry about using 3rd party batteries, hell I use them. You have to be high to pay Canon's prices. I bought a bunch of 3rd party batteries for my T1i @ $3 a piece.
Anyways, if you want to use 3rd party batteries, I would suggest you still use the OEM charger, if you can. Thats one less piece that you need to worry about. You're more likely to damage a battery on the charger than you are in your camera.


