My father is obese, and has been for many years. At the weight he usually hovers at, his BMR appears to be ~2,500 calories. He can lose weight from here no problem eating ~1600 calories a day, not easy, but not to hard.
The problem is, that after about 15 lbs of weightloss, he plateaus. Dexafit scans confirm there's been a massive reduction in BMR. By the time he's dropped 20 lbs his calorie counts generally need to be in the 1200s to lose weight, which is just torturous. The longest he ever last at these really low calorie counts was probably a year, I'm significantly smaller fitter than he is and I could not have lived a productive life on the diet he was eating.
I'm not sure how common this kind of response is, but his doctor's didn't seem to think it uncommon.