Please help! Annoying border on objects.

I have a problem, that I really don't know how to solve, or what I did to make it happen... 😞 When I use the shape builder tool, or simply paste one shape on top of another, there's a line coming out from the shape bellow regardless if they are the same or are perfectly aligned. I don't know if I'm explaining it correctly, but it's really annoying 😞 I've attached a picture, thank you all in advance.