no,
you are incorrect. Its an accepted phrase in the English language. I'm surprised none of the pedants here haven't hauled you up on your comment. It goes along with falling ill and falling in love. Though the image of someone falling down the stairs naked and landing on a cock is amusing...
some relevant definitions of 'fall' to help you out:

to succumb to temptation or sin, esp. to become unchaste or to lose one's innocence.

to pass into some physical, mental, or emotional condition.

to come by lot or chance

to come by chance into a particular position.
(lazy killer robot found that beer tastes better when your older, Wed 1 Oct 2008, 4:02, 2 replies)

*belms*
the reason the pedants haven't hauled me up on it is probably because they were smart enough to realise I was exploiting a popular phrase for the purpose of attempted humour.

I bet you're one of those people who whinges about puns.
( crackhouseceilidhband Fuck off back to Mumsnet, Wed 1 Oct 2008, 6:19, closed)

I read that
as
"to come a lot by chance

to come by change into a particular position"

I'm just wondering how repeatedly you'd have to change into that particular position before you came- and what the odds are for someone spontaneously ejaculating while at work...
( uberdeity Wondered why Owen Lars knew of the term "hell" on, Wed 1 Oct 2008, 10:58, closed)