is it normal to be able to get 16 rounds into a preban 15 rounder?
the 16th one wasn't any harder to get in than the 15th, but there's no way a 17th would fit

sometimes you have enough room for another, although it probably means that you're over compressing the spring.  There are some MecGar mags that could hold 17 rounds.

This issue of being able to fit more than the stated capacity is why the 10 round pre-ban mags are so tight for that last round.  They were being very careful not to accidentally make an illegal 11 round mag.

You can actually cut the spring a bit and fit 17 rounds into the 92 mag but it swells the sides of the mag and will definitely lead to early spring failure, Try loading the mag into your pistol with the slide forward and 16 rounds in the mag...it will probably not go due to there not being enough "give" in the spring to allow the mag to seat. For serious use, make sure you do not load more than 15. (actually, I would load 15 and chamber the first round so that I had only 14 in the mag, but I'm a belt and suspenders kinda guy)

Bad idea. Try to put the magazine into the weapon with the slide forward. Not to mention you are over compressing the spring which is the leading cause of magazine failure.
This is of course, for factory magazines, there are a few aftermarket that will allow additional rounds.
-Steve

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.

I used them all at the range with 16 rounds, since I had already loaded them before going, but in the future I don't plan on putting in 16