I recently heard of cases where the 380 failed to penetrate the sternum !!
Assuming this story isn't entirely apocryphal, I have a very good explanation.
First of all, let me say that the idea that 90 grains at 1000fps won't go through one of the thinnest bones in the body is absolute bunk. This kind of stuff is only accepted as truth by people who WANT to believe it.
BUT, there ARE situations where bullets SEVERELY underpenetrate, and there is a very good reason in many cases why that happens. To explain, let me tell a short story I read some time back in a news article.
A married couple had an argument which finally reached the point that the woman retrieved a .38 revolver and emptied it at the man. He was hit several times but was unhurt, and one of the bullets which hit the glass of a nearby window failed to break it.
So, do we immediately start posting on the internet that a .38 revolver bullet won't penetrate glass? I guess that's what some would do. It turns out that the couple was quite elderly and the revolver was one that they had purchased AND loaded MANY decades earlier. The ammo (which may have been old way back when) had degraded to the point that the bullets barely had enough oomph to exit the barrel.
Ammo is degraded by exposure to heat, cold and moisture. Long exposure increases the effect. I recently talked to a local gun shop owner and asked him about some ammo he had that I had not seen on the shelves in many years. Turns out he had been storing it in a metal shed with no climate control for well upwards of 10 years. In TEXAS where the summer temperatures in a metal shed might climb well past 120 degrees, the humidity is oppressive and the winter temps sometimes reach below freezing. I didn't buy any, but I wonder how many people will...
Now consider that many of those folks fit into the group that buys a gun and a box of ammo and figures they're set for life. The gun gets loaded and goes under the seat of the pickup truck and stays there. Then, 20 years later after going through the extreme temperature swings experienced by a car in this area, the guy pulls it out to use it and blames the caliber because the ammo doesn't penetrate enough.