The dipstick!!! That's a damn good possibility!

However, its not the pressure to lift the dipstick that I am concerned about. You might develop 3-4 psi in the crankcase with your thumb. This is more than enough to lift the dipstick. However, you must now multiply 4 psi by the total number of square inches inside the oil pan to determine the total force that is trying to force the pan off the bottom of the block. There are probably 100 square inches inside that pan and 4 psi would create 400 lbs. of force trying to blow the pan apart. This might be perfectly acceptable to the pan, but, nobody really knows for sure, except those that have successfully done the test.