It might be a Tiled Background wrapping around the other side of the image, or it might be spritesheet bleed from an adjacent image. As ever it's hard to say for sure without seeing a project file. If it's the latter, changing the downscaling quality may help.
I don`t have very big wanting to share my project with over 300 objects and over 2000 actions, but I have seen this bug even in new project sometimes, when at least one object exist.
I have croped the object just after meeting this bug, but this gave no result. Also I am not shure that setting a higher quality is a good practice because I have a really big project now and I am going to build it for android, so high quality downscaling would kill perfomance in the game.
I feel myself a little bit stupid. Is ther any other ways to fix such bug?
P.S.: Do you recommend me an option, which was labelled by yourself as "not reccomended"? :D