In this comment on one of the answers to an earlier question of mine, @CarloFelicione points out how even the tiniest of imperfections can make a stealth aircraft much more visible to radar:
A screw not properly seated on a maintenance panel can make the plane show up like a barn door on a radar. Not the size, it’s the reflectivity that matters
How is it possible for a single loose screw to increase a stealth aircraft's radar cross-section that dramatically? Why does it show up like a barn door, rather than like, well, a screw?