It seems like I am always putting in the wrong side of the battery when I am replacing batteries in something. One time I stuck four rechargeable batteries into a battery pack and plugged it into the wall.
I didn't pay any attention to the light or even realize that they didn't go in very easy. The next day when I went to get the batteries, I was frustrated that none of them were working.
It was not until then that I realized I had put the positive battery side in the wrong way on every one of them.