I had that happen when I was using a lavatube. I used a corkscrew to push the dent out (kinda-sorta) from the inside.
It's caused by dropping your mod or having it roll off a table, that sort of thing. The battery compresses the spring when it lands, then the spring presses it back, for instance, assuming it lands right side up. Or if it lands upside down, the battery itself is trying to force itself and the pin is there. Either way gives you this result.
I didn't worry about it much - but I do prefer button top batteries. I'm using a provari now, with AW and Efest button tops, and I no longer have the problem.