No, it didn't approach 80 at my house yesterday. This is showing a problem with where I put my outdoor sensor.
It turns out that putting a black sensor (DS18x20) somewhere that it can pick up direct sunlight is a bad idea. The sun poked through the clouds for the first time since I put the sensor out and immediately spiked the readings.
I'd originally put it in a more sheltered location (though it would still get sunlight for a few hours), but moved it well out into the open to get it away from the back door. Every time we opened the door the warm air would raise the readings, keeping us from knowing just what the temperature is out there.
The sensor really needs to be shaded, but at the same time away from doors and windows that will let heat out to affect readings. I think I'll be moving it to the north side of the house as soon as I can figure out a good way to mount it over there (it was just sitting on the patio table up until now).