I think I did not follow instructions on how to ask questions initially. My question is:
Why does my toilet tank overflow when I flush it? My toilet tank overflow overflows when I flush it. I keep getting responses leading to the flapper. How can this be? I have had to turn the water off in that bathroom.:o

If your water level is high the water will over flow through the overflow tube, you need to adjust your water level, see the images below and tell me which fill valve do you see inside the water tank, Thanks.

John

John is right, your flapper is working all too fine but the fill valve is not shutting off or is not adjusted to the correct water level 1" below the overflow tube. Right in the center of the black and white diagram is a part called the "water level adjustment clip" squeeze the clip and then slide the rod up about an inch. Did this help?

Bob is right, if you have Fluidmaster 400A fill valve, just squeeze the clip and slide the float up about an inch, flush the toilet and wait until the tank is full and the fill valve stops, check the water level it should be 1" below the rim of the white overflow tube if it is more or less adjust it accordingly and make sure the small refill tube is connected into the overflow tube.

If you have the Ballcock type fill valve then you have to play with float arm bend it down or up see the Image below, good Luck.

John