Now and then this is happening with this particular file that Iam working on… What are those lines? I didnt accidentally messed up with the geometry, they just appears…I think there is something to do with hiding layers and making intersection in the faces… Anyone could help me?
My guess is that you have done just that, and have moved parts of the geometry, and the rest has autofolded and created those edges. As has been said many times before, hiding layers or hiding loose geometry doesn’t stop things sticking together.
You will need to attach your skp model for a correct answer. If it’s too large to post directly you will need to use a file sharing service and post the link here. Without seeing the model we are just guessing as @Anssi says, but his guess is probably correct. If you are using the term “hiding” layers (tags) it implies you think that somehow protects the geometry from being moved / modified. It does not.
I never asked that question before
so everything must have a group to prevent this to happen with hiding layers
Correct. Groups and components are the ONLY way to isolate geometry and prevent it from interacting (being sticky). Geometry that is hidden or not visible via tags (layers) still gets stuck and moves around, you just can’t see it happening.
In a well organized model every discrete object is a component or group. There should be very little to none “raw geometry”.
Then you can assign tags to the Components and groups you have made to control their visibility.