There are a few limits that need to be adhered to when using Flashlight.
Edge size. The minimum edge length (rectangle side length or collision polygon side length) must be bigger than the Flashlight’s diameter. If the edge is shorter than the diameter then if it’s normal was pointing at the light, then (depending on the shape) both sides could be illuminated by different parts of the light. The Flashlight object does not accommodate this situation.
Minimum vertex angle. The minimum corner angle that is supported is 90 degrees, although in most situations angles that are less than 90 degrees will not cause a problem. Incorrect shadows can get drawn from very sharp / small angle corners when they are pointed towards the Flashlight object. This is a limit of the plugin because of how the shadow casting edges are determined and how their shadows are drawn.
Object shape. Only convex shapes are supported.