(Note: I want to add an example to a definition that I submitted and that's already been published, but I don't know how to do that. Below is the original entry with the new example.)
When one person is trying to get around another whose facing the opposite direction and, thus, cannot see the first person trying to squeeze by without touching. At the last second, as the first person is about to pass, the second steps directly in his or her path, resulting in inadvertent crotch-to-ass contact.
After an unintentional moon landing, the first person might say: "Oops! Pardon me! Didn't mean to land on your moon."
"That's one small step for man, one giant dick in the ass!" (new example)