If a gun fires when it is dropped, that is called a drop fire. Handguns are meant to be safer than ever these days, but consumers are still reporting a large number of drop fire accidents.
Sig Sauer has been sued more than once due to drop fire defects, and large settlements have been reached by injured plaintiffs. The Sig Sauer P320, sold to the US Army, was determined to be defective as it fired when dropped at a certain angle. The gun is also popular with police departments and consumers. There have been several reports of the gun firing when simply being holstered.