Never let the bare board of your mobile battery touch the bare board of your Raspberry Pi!

Today while I was designing my device enclosure, I accidentally let the bare board of my mobile battery make contact with the bare board of my Raspberry Pi. The moment they made contact, I heard a "pop" and then smelled a burning odor. I then quickly checked the operation of the battery and the Raspberry Pi. Fortunately, the Raspberry Pi was fine, but the battery was dead. I had to buy a new one.