Strange Shadow Forms on Player

This bug appears to be entirely random, and therefor very hard to recreate, but sometimes I notice that when moving around, the item in my hand turns very dark. One time it happened while I was in third person, and I noticed the entire player turn black for a split second. It only happens for a very small amount of time (maybe only one frame), but it still happens. This bug is not very important, nor is it easy to recreate on purpose (or sometimes at all), so please don't waste too much time trying to fix this.