Just spontaneously? Or are you concerned with someone dropping a fork in it, and shorting it out? I've just never heard of anyone worrying about a toaster. Christmas tree lights, cable boxes, or small fans, stuff that you leave on all the time, I've heard of, but never a toaster. Now what do you wanna bet, my fuckin' toaster will go up in flames today?!
All of the above.
I think I've watched and (re-watched) too many Final Destination movies where ANYTHING can happen.