OK, not exactly a disaster but it's weird... I clean my wash basin and bath drains by periodically blasting mains pressure water (via a hose) down the plugholes. Very quick, no chemicals and it does a great job. Done it a good few times and if careful, doesn't spray water around the bathroom. Last week, I thought the wash basin drain looked grubby so I gave that a blast too. After a few seconds, water began to run out of a small drain hole on the underside of the basin! WTF? Turns out that the basin overflow must somehow/somewhere be open to the ceramic washbasin void, i.e. the air space between inner and outer basin surfaces, and I had managed to fill (well, up to the level of the void drain hole - so about half fill) the void space with water. Obviously, I couldn't leave the water as it will soon 'go off' and cause odours. So, I carefully threaded a thin plastic tube through the small void drain hole and teased it into the area beneath the bowl (did it by feel as I couldn't see a thing). When I thought the tube was in place, I blew into it and heard bubbling water - proof that water now part filled the void. I managed to get a siphon going and removed over a litre of water. I think I had emptied the void but small amounts of water may have remained so I borrowed a pond aerator pump and set that up to blow air into the void for a few days. Nearly a week has passed and no odours as yet. Fingers crossed it stays that way.