Do you guys let someone know if their fly is down or do you let them go around with the barn door opened? If you do tell them, what do you say?

Here's the top 20 that I found online:

20. The cucumber has left the salad.

19. I can see the gun of Navarone.

18. Someone tore down the wall, and your
Pink Floyd is hanging out.

17. You've got Windows in your laptop.

16. Sailor Ned's trying to take a little
shore leave.

15. Your soldier ain't so unknown now.

14. Quasimodo needs to go back in the
tower and tend to his bell.

13. Paging Mr. Johnson... Paging Mr.
Johnson...

12. You need to bring your tray table
to the upright and locked
position.

11. Your pod bay door is open, Hal.

10. Elvis Junior has LEFT the building!

9. Mini Me is making a break for the
escape pod.

8. Ensign Hanes is reporting a hull breach
on the lower deck, Sir!

7. The Buick is not all the way in the
garage.

6. Dr. Kimble has escaped!

5. You've got your fly set for "Monica"
instead of "Hillary."

4. Our next guest is someone who needs
no introduction...

3. You've got a security breach at Los
Pantalones.

2. I'm talking about Shaft, can you dig
it?

AND THE NUMBER
ONE WAY TO TELL
SOMEONE HIS FLY IS UNZIPPED...

1. I thought you were crazy; now
I see you're nuts!

Yes i tell them

If it's someone I know, I just quietly tell them to check their zipper .... Just one of those things that happens. :)

I tell them. I usually just say your fly is open or XYZ. Which stands for examine your zipper. I hope they would do the same for me.

Yes I tell them.  I usually motion downwards and then ask them.  " What do birds do ? "

At my work Superior Court of CA. I happened

to notice that a clerks zipper was down.

So I whispered in her ear, Zipper, then she smiled,

and looked down and said thank you.

This can be done, but be tactful.

I will whisper to them that their zipper is down.

I usually give a small "sorry to tell you this" smile and point down, 

"XYZ" is the one that I've heard.

the only one i knew until now was " your post office is open", thats all ive heard being used!