Just replaced an old pedestal sink, and had good success following advice on this forum to stop the leak around the drain tail rubber mack gasket -- a little dab of silicone grease and an extra 1/2 turn using 3 fingers at once seemed to do it (luckily no threads met the gasket to daub putty into).
On the other hand, I used rigid copper water supply lines, and they both leak right where the bull-nose "plug" goes into the base of the faucet assembly. The leaks are tiny, about 1 drip every 5 hours or so (no, I didn't time it ). The big box "expert" claimed that once failed these types of bull-nose fittings simply will never work, so you have to replace them. I bought two new ones, re-cut, re-bent, re-assembled, re-cursed the painful working space of a pedestal sink, and ... same exact leak. I didn't bother to install the 2nd one.
Is there any validity to the statement that once they fail to "seat", these types of fittings just won't work? I've tightened them very tight, but it's rather awkward to get two wrenches up there, so it's possible I could get a bit more. Should I try to tighten further? Would silicone grease on the bull-nose tip help? Should I just ignore it and assume sediment will eventually fill in whatever microscopic crevices admit the tiny flow? Any other suggestions (other than going for braided flexible or PEX supplies?).