Ahh the mistakes made growing pains and lessons.
Remember that little piece of plastic that covers the view finder we normally just chuck out? Known as an Eyepeice cap, it covers/blacks out the view finder and why one might ask?: Well here are a couple examples: Needless to say during a long exposure, light leaked through the viewfinder causing the color banding!
This actually applies to light leaks in film cameras as well, be they 35mm, medium or large format cameras; mirrorless, however, maybe not since the view finder is an EVF!
Lesson here is: When provided by the manufacturer an item will usually have a use.