It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water---with which it reacts explosively. It can be kept in some of the ordinary structural metals---steel, copper, aluminium, etc.---because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.
—John Clark, describing chlorine trifluoride in Ignition! An informal history of liquid rocket propellants, quoted by Derek Lowe in "Sand Won't Save You This Time".