I blush ALL THE TIME.
Seriously. It's mortifying. I don't even have to be embarrassed or thinking dirty thoughts or anything. Just do something unexpected and watch me turn red. A few days ago someone said hello to me in a grocery store and I turned 7 shades of crimson withhout even realizing it! When my girlfriend pointed it out to me, I was just as surprised as she was. lol
I don't understand why, but I do blush all the time.