That's a tough one because, as an instructor, you can only check so much. Who would ever think of checking the spar for proper glue.. how would you do that, anyway.

One thing I learned from my instructor was to slobber in the epoxy until you can't get any more in (the spar slot in the wing) then push the spar in. The glue oozes out so it's just wiped off then the wings are joined, aligned, and the glue allowed to set up.. I let them go for 24 hours.

That must have been really tough to see that fine plane go in like that with that helpless feeling you get when you know it's to late to do anything.

CGr.