What causes toilet to overflow without anyone using it?

Question

Every once in a long while, we would leave home in the morning and come back in the evening to see our bathroom floor totally flooded. In the 5 years I've lived in our house, it's happened 3 times, and each time my husband and I would scratch our heads and wonder "what happened?" because we didn't do anything different, and the toilet was working fine when we left it. The water is clear and doesn't seem to be coming from the sink/faucet because the area around the sink is dry. So we are guessing it's the toilet. But what would cause it to overflow like that in the middle of the day??

Answer 1

That happened to me too. It turned out to be a sticking arm inside the toilet. The flapper would leak and the toilet would refill and the arm inside would stick in the refill position. It overflows from the top of the tank. Check The guts of your toilet tank.  

Answer 2

I wouldn't usually guess at answers, but this question appears a little familiar to me. I would guess that you live in a rural area., because this is the sort of thing which occurs where I live when it rains very heavily. For that reason I would guess that you live in a geographically flat area. If it rains for a prolonged period of time, we can get a back surge of water through the toilet, and with a lot of gurgling. It hasn't yet overflowed on to the floor. For a short time after, when we flush the toilet, it will fill up to the top, and then subside slowly.  RobertS above, seems to give a very probable and more reasonable answer.