Such things do happen. A box of ammo spills and something gets mixed up.
First off you will likely see or feel feel the difference in the rounds in your hand as you go to load them in a magazine. Or you may note that one round in the box is a good bit shorter than the others. If you don't notice this...
You will likely notice that the .380 rounds do not fit in the 9mm magazine as they should. You can try it right now and see what the difference looks like...see once you see it it ain't hard to miss.
At this point most shooters will go "Is this right? What's up with this round?" If not, ignore it, have another beer and shot to match the ones you've already imbibed and continue.
The .380, cuz it is a less powerful round than the 9mm, will likely not damage the gun when it goes off but, can and will likely cause a jam, being shorter and all. Feeding, properly chambering, extraction will all be problematic. A few may feed but not many. This will be a clue to the sober.
You can also lesson the likelihood of some mix up by never loading a round by dropping it in the chamber (I don't know why some fellas do this, it gets you nothing) load from the mag.
tipoc