This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
A lot of people seemed to think that there was a mysterious
third person in the bathroom when Bobby and Diane had sex near
the end of the fourth season finale "A Draining Experience."
For the confused, those mystery hands peeking over the stall
actually belonged to Diane; various folks have dissected the
scene enough in slo-mo to assume that for her hands to be in
that position, Bobby would have to be making a rear entry of
some kind. I'll leave the rest up to your imaginations. :)