The bottom of the soda can is
concave. (The bottom of plastic bottles is
convex with a false bottom to make it stand up.)
The sides of the can are (gasp!) not flat either!
They are also round--convex in this case. Milk cartons
have flat sides. Soda cans do not. Also note that plastic
milk jugs are frequently not flat either, but have some
kind of indentation in them to increase strength.
The bottom of your propane tank pops from concave to convex
immediately prior to catastrophic failure, just because
the force to pop the bottom is less than the force to
burst the tank, and so that will usually occur first.
I don't think it has anything to do with a built in warning.
If there was such a thing, it would be in the form
of a relief valve, like what is on most liquid air tanks.
Plastic soda bottles probably have a convex bottom because
the pressure needed to collapse a plastic concave bottom is so low
that the fizz in the soda itself would do it immediately.
Jhonbus's explanation below of the bottom sounds right to me.