My TF2 wanted to validate many times, but each time, it seemed like it wasn't progressing. So there were several times that I canceled out of it, even after it said it was at 100%. Steam would try to do the validation thing again sometime later, and the headache continued. I decided to just let the validation run until it completed itself, which took longer than it should have. Finally it gave me the prompt that the validation was completed.
I haven't had any more issues with it since.