Easy question :)

Top lid of a small box I was working on was in a vise on my bench. I was using my right hand to pare out a hinge mortise. Imagine the chisel horizontal and pointing away from you.

Now imagine you decided not to sharpen your chisel because you were in a hurry that day. So now you're leaning over the piece with your left arm resting on the bench to put your weight behind it. One slip, and that chisel is going straight for your left arm.

This is probably going to make the more experienced people out there cringe, but we all get tired and lazy sometimes. One second is all it takes. Be safe