Shortly after defeating Grey Fox and meeting up with Hal, the player will have to find Meryl, who is currently disguised as a Genome Soldier. When the player finds her, she will run off into the women's bathroom and change out of her disguise.
If the player follows her quickly enough, the next cut-scene will show her without any pants on, as she didn't have enough time to change properly.
However, if the player goes into the bathroom before seeing Meryl and calls Mei Ling on the codec, Mei Ling will chastise Snake for going into there.