Undershirts.

Most people wear undershirts to avoid sweating through a tshirt they actually like.

There are only a few things more awkward to look at than someone who has poorly matched the tshirt he or she is wearing underneath another tshirt.

What I mean is when a dude is wearing an undershirt that is BIGGER than the shirt he's wearing over it. You get that goofy-looking overflow (or underflow?) that makes it look like there's some Austin Powers ruffly stuff going on at the end of the sleeves and the bottom of the shirt looks like the tiniest skirt. Not to mention the very noticeable lines from the undershirt folding over itself.

Maybe a look in the mirror. Or even looking down when you put a belt on.