Well my favorite trick is to set the root rib perfectly on one wing panel and then set it backwards on the second wing panel making a joint that produces a perfectly flat wing when in fact it is supposed to have dihedral in it.
You would think that doing this once would be enough, but alas it has happened twice and neither time did I discover it until it was time to join the wings.

And just to finish this off one time I managed to get both hands glued to the airplane and my elbow glued to my thigh through my jeans .... yeah yeah I know it sounds like the old Tim Taylor Toolman skit .... but I did not call out for the wife to come and help, skin and balsa are pretty close in strength, the plane had skin stuck to it and my fingers had balsa stuck to them .... and just in case you were wondering fingernail polish remover will cause a rash on your thigh before it softens the CA enough to get your elbow off your jeans and your jeans off your thigh.