I had an issue with this last fall - had my AC go out in the fall when it had been over 80F for the last few months. Outside, it would be 70 during the day and 50 overnight, but there was so much heat in my walls/floors that I literally couldn't keep my apartment below 80. If I opened every window and door, I'd get down to 75 or so, but it would be back to 80 within an hour, even when the temperature outside was significantly cooler.

It's great in the winter though. I haven't had to turn my heat on in years.