Leaking Shower Head pipe

[JeffD]

I am remodeling my current shower, exchanging the current standard brass coloured fitting for polished chrome. I exchanged the shower head pipe. I even put PTFE tape on the new pipe threads. Unfortunately the threads that I am screwing into are behind the tile wall with only enough space for access for the pipe. Anyhow the fist time the shower was used in anger a load of water started to leak into my dining room from the air conditioning vent. I had only hand tightened the pipe so that didn't ruin the threads. Any ideas what caused the leak and how I can rectify it.