There's a little extra slack in the fully-loaded mag so that it'll fit in a gun with the slide closed. Sometimes there's just enough extra slack to fit another round, but the mag definitely won't fit into a gun with a closed slide like that. Like Hoplophile said, the Clinton mags are extra tight on that last round to make sure that it'll fit in the gun, but not take an 11th round.
What you can do is lock the slide open, load 16 in the mag, stick the mag in and drop the slide. Now, you have your full 15+1 without taking the mag out to add an extra round.