I wouldn't be surprised if the unibody frame bends when removing a case. The weakest point of the frame is the tiny cross section at volume button hole.
Like in the video (
@0:41) above, one may have the phone face down and pull from the corners and push from the middle. If done first on the volume button side (video suggests correctly to start from microsim side) there may be enough leverage to cause high stress to the cross section at the lower volume button hole. It
could be that the required finger force isn't that high or doesn't
seem excessive.
I tried to analyze this load case with an approximate FEA model. As the model doesn't have absolute exact dimensions, it should be regarded only as an estimation (the model is made using the information from the published iPhone 5 external drawing and some photos from internet). Also, what's not exactly known is if the unibody material is heat treated or not.
Pulling from the corners and pushing at the middle with 50N force (= 5 kg = 11 lbs) may result a stress comparable to yield strength of non-tempered 6000 grade aluminum. It's an estimation, but
could be in the ballpark.
Red area at volume button hole edge is the highest stressed point at 55 MPa (8 ksi).
With bathroom scale it's easy to try how 5kgs (11lbs) feels on thumbs. Some one dare to try with iPhone 5 (I don't have and no one wants to lend me one, wonder why
)? I tried with couple of other phones (including 4S) and they could stand it ("tortured" my L800 up to 12kg). What is excessive for a phone, that question of course remains.