So I've seen a lot of posts with people worried about guests wearing too casual attire at their receptions.
I just had the opposite problem crop up.
A guest just mentioned that he needed to get measured for his tux.
I responded with a "haha" because he knows damned well that it is not going to be held at an elegant venue.
Based on HIS response, though, he's seriously planning on getting a tux. (BP will be wearing tuxes, no one else.)
I've let that go for a full 24 hours because I have no idea on how to politely tell him "Don't fucking do that!"