This sounds very similar to a problem I have keep having. Water from
an upstairs bathroom is leaking through my dining room ceiling
everytime we use the shower. I have a Delta shower/faucet on the tub.
The tub spout has a diverter that you activate by pulling down. When
water flows from the spout into the tub no problem. When you pull the
diverter, the water comes showering into my dining room through the new
hole in my ceiling. The pipe the spout fits over has a brass nut on
the end. There is a nylon piece between the nut and the shower wall.
The nylon piece moves back and forth on the pipe, but it looks like the
brass nut is supposed to hold it in place. This piece has nylon
threads for the spout and a rubber gasket. I held the nylon piece
against the nut (so it would not turn) with my finger (about 1/2 inch
from the wall), then I screwed on the spout. I stopped when it was
about 1/2 inch from the wall and turned the water on and then the
shower. Water came out from the back of the spout - this is clearly
my leak. The water goes through the hole in the wall where the pipe
comes through and falls directly down to my dining room. My question
is this - if I put teflon tape on the nylon threaded piece and replace
the gasket, should this solve the problem? I was also thinking of
putting silicon sealer around where the pipe comes out of the wall, so
that if it does leak it won't go in the wall and further ruin my
ceiling. I am so frustrated with this thing.

I thought I had this problem fixed once before, so I repaired my
ceiling only to have it ruined again. For everyone's info, I called
Delta and they said this thing has a lifetime guarantee, so they are
sending me an all new spout, but that will take another 2 weeks.