I'll admit....I did the same thing once myself. I went at least 2 weeks before I noticed it. It was on my passenger side so I never saw it walking up to the car. I acutally didn't find out about it untill I went to sell my fronts for a pair of rears. When I dropped off my wheels after the tires were taken off you could see the 1" difference. I will say the guy at the tire shop didn't notice it either. Neither did the guy at the shop where I bought the other set of wheels. Point being is that it's not super obvious on the width unless you're specifically looking at it.
I ended up putting on the 20x9 all around and there has been no issue. I never even felt the smaller wheel/tire on the rear when driving and haven't seen any kind of damage. That was well over a year ago and this is the first time I've admitted it either. It's not something to be prowd of.
Things happen....your car should be fine.