For that same reason, you should never spray WD-40 on a squeaky engine belt. Like your brakes, accessory and fan belts need friction to work properly, and WD-40's ingredients can damage the rubber in the belts. A squeaky belt is usually either a sign that the belt is not at the proper tension, or that one of your pulleys or engine accessories needs to be replaced. And while WD-40 is good at removing the glue from stickers and decals from glass, you should be also very careful when using it on the inside of your windshield. The solvents in WD-40 are dangerous if inhaled, and if you use too much, it could drip into the defroster vents and be forcibly blown at your face later on. WD-40's Material Safety Data Sheet warns that this could cause "anesthesia, headache, dizziness, nausea and upper respiratory irritation."

You should also be very wary about using it on a hot engine. WD-40 is highly flammable; in fact it can be used as starter fluid. It dries very quickly, but if it makes contact with a hot exhaust manifold, pipe, or catalytic converter, a dangerous and destructive fire could break out quickly. The MSDS specifically cautions that it should be "kept away from heat, hot surfaces, sparks, [and] open flames." The warning about sparks also means you should never spray WD-40 on battery terminals, where arcing is likely to occur.