How can a single misplaced screw so massively increase a stealth aircraft's radar cross-section? 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?
In RCS, size does not matter once you move beyond the scale of the wavelength of the radar. Therefore, the RCS of a panel with an edge length of 10 cm is (theoretically at least) the same as one of a panel with 1 m edges. The screw must be so poorly placed that it will cause a larger panel to stick out so it has no electrical contact, with the edge pointing in the right direction, then this massive increase in RCS is indeed possible.