When I'm moving right along on a build and discover a mistake I made. Like as of this moment!!!!!!!!!!![:@]Nothing I can't correct but I really hate it.
I don't like building wings all that much, it just takes a lot of time but I think the hardest part for me is installing and setting up the radio gear, servos and what not. I make up all my control rods and have my planes operating before I ever start covering or painting/glassing. Just so much time involved. Kits I don't mind the wings as much but on a scratch/plans build it involves a lot of thinking and time, fitting every part before gluing.