page from Iron Man Vol. 2 #6 showing the hulk confronting Mr. Fantastic, Captain America, and Sue Storm, Sue has her back arched and butt thrust out with both butt cheeks prominently outlined in her tight pants

When Pym Particles Malfunction

I think the Pym Particles are malfunctioning on Sue's suit and shrinking so tight it's pushing her in that awkward pose and letting us see the definition of both her buttcheeks and her nipple.

(Pages from Iron Man vol. 2 #6, Marvel Comics, submitted by anon)