A picture is worth 1 kWords.
This one is from retired KFI/KOST Chief Engineer Marv Collins, who says that while working on a piece of battery operated equipment he couldn't figure out why the gear wasn't working. "Then I noticed that the 9 Volt battery polarity was labeled in reverse.
“It took me a few seconds to believe what I was seeing,” he continued, sending along an image of the Duracells in question. “I wonder how many of these batteries were manufactured and sold before the mistake was discovered.”
Marv adds: "I should mention that had I been using a normal 9 Volt battery connector, there would have been no problem. But I was using clip leads and I connected according to the PLUS marked on the side of the battery. The battery case was put on the battery in reverse, which fooled me."