It did it on my roadster a few weeks ago...gave my passenger quite a shock with a very wet leg. Not helped by me operating the washer again just to check!
The rubber pipe is quite fiddly to push onto the T-piece as you can't get a lot of purchase. The best thing as Dave says is to drop the dash slightly then at least you can get a grip on the T-piece and loose tube and see what you are doing!
I wonder if this is a common problem? The factory now fit an alloy plate behind the instruments on the new Roadster to stop water getting into the expensive electronics!
