It was dropped and the R top corner was damaged. Distorted enough that all three layers of the case were deformed. The bottom case half had a widened 1/4 inch gap along the elongated vent , the top had a 1/4 inch diameter ding that looked like a spent bullet. Smashed and mushroomed. The screen aluminum cover had what looked for all the world like stretch marks. Since I did not have apple care plus (i have apple care, but this impact will likely result in denial of any future warrantee requests: "You obviously dropped this". The laptop has worked flawlessly but the reality of the world will be to exclude on that basis.
Ordered a replacement case bottom and a tool kit, the professional took kit with miriad little bit heads which interestingly enough did not contain the one that exactly fit the screws on this particular Pro(?) resulting in one of the screws getting chewed. If you've used a too small Phillips head screw driver you know what i mean. The damage required me to use a dremel to carefully route out space to allow fitment of the replacement case bottom. There exists tools referenced here on the iFixit blogosphere to reshape the corner, but their cost and the necessity to further disassemble the laptop to use it properly made me choose the dremel. Then a file to smooth the mushroomed aluminum made what had looked like a badly decayed molar almost unnoticeable.
Don't drop your expensive aluminum laptop, don't drop even a key onto it from any height further than 5", it will make a mark and that mark will be permanent and not easily or inexpensively repaired. Dear Apple, please start making things out of materials that are more durable. The aluminum looks great but is easily damaged. My machine was dropped from a height of about 1 foot.