Done this twice now. Never ever never put a half loaded mag back in a mag carrier as you unload and show clear when completing a course of fire. This recent time had no ill effects, Shooting a classifier that was shoot 6, reload, shoot 6, reload, shoot 6 weakhand, I did. And on the last weakhand round the slide locked open with a 10 rd mag installed. Imagine my surprise. Least I didn't lose any hits.
The other time was in a 40 some round field course, mag felt light, musta tossed it 20 yards as I grabbed another one.