tert-butyl lithium. Ignites on contact with air. Often used in conjunction with flammable solvents, so large fires and explosions are possible when working with large enough quantities.

As far as safety SOPs go, nearly any chemical spill of a large enough quantity warrants evacuating the area in my chemical safety plans. For some institutions, this is as little as 1 liter or 500g of material. This can obviously be overkill if you spill something that is relatively inert and non-toxic such as water or NaCl.