Yikes, my Yorkie never did this... yet. (Knocking on wood that he doesn't) But our older dog did when he was a puppy. My husband is in the Army and he came home for lunch one day and was laying on the floor playing with the puppy. My husband was on his stomach and Cosmo climbed right up his back and just let loose! I couldn't figure out if he thought my husbands uniform (BDU-camo) was grass or if he was just being a turd!
Needless to say, my husband was certainly not thrilled, although it was really hard for me not to laugh. It never happened again, so hopefully in your husbands case it was just a one time thing as well!