I've found a bug with copy pasting a craft. I was working on a shuttle craft and a 747 launcher for the weekly challenge. When I went to connect the shuttle to the 747 craft I did a copy paste first and tried attaching the copied craft to make sure I had an original if anything messed up. On the copied craft I went to adjust the existing fuel lines and the struts and found that when I clicked on one end of strut/fuel line it would disconnect the end from the copy and jump to the original craft as if I was editing that one instead of the copy with one end attached to the original location.
I would then have to reattach the strut/fuel line to the original craft and then go back to the copy and add a new strut/fuel line to replace the one that got removed.