My bike has Ultegra components and low gears slip when climbing. I'm not sure of the exact teeth but I'd say that teeth 2-4 (going from low to high gear) on the rear cassette slip. It sometimes slips on flats but it's definitely noticeable when climbing. It's to the point where I'm either on gear 1 or gear 5 to avoid the gears slipping.
I thought it might be a bad cassette but the old cassette was only about 4 months old. I bought a new cassette anyway but it's still slipping at the same place. I suppose it could be the chain but the chain is only 6 months ago.
I did remove the rear cassette to install it on a new set of wheels. Is it possible for me to have messed up the teeth with the chain whip when I was removing / installing the rear cassette?