Flat Top Vs. Button Top
To really demonstrate how non-standardized the 18650 battery is, there are two slight variations in size: flat top and button top. This relates to the contacts, specifically the positive contact. Button top batteries will protrude slightly, whereas flat top batteries sit perfectly flush.
These extra few millimetres can be the difference between a battery that fits and a battery that doesn't. If in doubt, look at the existing batteries that came with your device, consult a manual, or contact the manufacturer. For spring-loaded batteries, like flashlights, it shouldn't make a huge difference.