A firearm isn't all that complicated, but it's very important for every part to work properly. Remember, each time you pull the trigger, the bullet is propelled away from the gun by an explosion that takes place just a few inches from your hands and face. That's definitely something that you want to happen properly to avoid injury.
One malfunction that is very serious is known as a squib round. When the gun is fired, a squib does not exit the barrel, but gets stuck. Much of the time, this means the round is totally invisible, and shooters will not understand what malfunction they had. They may shoot again, with the next round up, and fire that bullet right into the one that is stuck.
A squib can cause the entire barrel and chamber to explode. Videos of these accidents show some guns being torn completely in half. The amount of damage depends on many factors, such as the size of the gun and exactly where the round is stuck, but it's quite dangerous no matter what.
Now, squibs are often a problem for those who reload their rounds. If this is done improperly, the round may fire, but without the force it was meant to have. However, this can also happen with factory rounds that are poorly made and still got through the quality control process. If a gun's barrel is improperly forged, it could cause a round to get stuck.
If this happens and you are injured in Illinois, be sure you know if you have a right to seek compensation for those injuries.
Source: Squib Loads, "What’s a squib load and how do I avoid one?," accessed April 19, 2016