I had this same problem the other day. I think it's caused by the holes in the pipe that are there to drain away the water coming through the overflow. With some sinks, the rubber gasket on the drain pipe rises up higher the holes in the drain. I fixed the problem by buying another rubber gasket and using it in addition to the one that comes with the drain. The two gaskets together were thick enough to stop the leak.

Hi pwykes and Welcome to The Plumbing Page. At AskMeHelpDesk.com. You're responding to a 7 year old dead thread so I gave you one of your own. Look in then upper left hand corner of the first post form the date before you post, Thanks,
You had a leak under the lavatory drain? (see image) Tightening the nut should have stopped the leak. Did you use plumbers putty or Silicon Jell for a drain lip seal? Is everything OK now? Back to you, Tom