HAH! I've just figured this out.
How on earth did the hassock get where I would trip over it? I wracked my brain, and then realized that I must have moved it where it was while I was vacuuming, then went on my merry vacuuming way and forgotten to put it back out of the way. Which still wouldn't have been its original position, but it wouldn't have been in my path either.
And how did I re-injure my knee??? I washed mini-blinds in my bath tub and then cleaned out the tub, and I was on my knees to do this.
So, I deduce that housework is dangerous.
That's my story and I'm sticking to it. No more housework for me. (fat chance)