Organic Chemistry lab --

Label everything, especially when two subsequent steps of your reaction look very similar.

If you're going to leave something stirring overnight, make sure there's a backup power supply, especially if your area has a history of power failures.

Not mine, but -- If the temperature of your oil bath seems to be going up much more slowly than usual, check to make sure the thermometer is working properly. Don't just turn the heat up until the temperature until the thermometer reads correctly. One of the people in my lab managed to cook his compound at 280 C because the tip of the thermometer was slightly above the surface of the oil bath.