Double check that it's not the large flange and rubber seal that's mated to the sink itself. It may be leaking there and then running down the tailpipe to give you a false sense of where the leak actually lies.

On Tuesday, April 2, 2013 8:12:44 AM UTC-4, Mikepier wrote:
I replaced a friends bathroom sink tailpiece yesterday and I am having
problems with a leak in the pipe with fine threads that goes into the
main body of the tailpipe. It is part RP 6128 in the pic below.
http://www.naturalhandyman.com/iip/i.../i/infpop1.gif

When I first installed the tailpiece, it leaked. So I removed it and
used teflon tape, still leaked. Then I used Rectorseal teflon pipe
seal, still leaked.

So I thought maybe I have a defective pipe. So I got a new one,put it
in, same thing, still leaks. Any ideas? I'm ready to put 5 minute
Epoxy on this thing if i can't stop this leak.