If you see the words 'physical reverse polarity protection' in the description of a light, it is pretty much guaranteed that you will need button top cells for it. As an example, almost all Nitecore lights that use Li-Ion cells; have a raised ring around the positive contact whose sole purpose is to keep the flat end of a reversed cell from making contact. Since that has the same effect on a flat top cell, a button cell is required. One way to get around that is to find a small magnet and let it fasten itself to the center of the flat top. For added security so that it doesn't slide and possibly short out the cell if a physical shock is applied to the light (dropping,etc.) I found some cell top insulators that are designed to be used if one is rewrapping a cell. They are similar to the hole reinforcers one can use to repair torn holes in notebook paper, except these were a little thicker. They serve to keep the magnet centered on the top of the cell.
Others will tell you that you can put a blob of solder on top, but it that is not done properly too much heat can be applied and that can damage the cell. Ninety nine point nine of the time button top cells can be used in any flashlight, so if you are not sure I would say get the button top version.
