Depending on what you use for air pressure you can spin it faster than it is rated to, and can introduce uneven load on barring. That is not the main problem, because it is usually not that big stress.
Every electro motor is creating force when you put current trough it, and when you put force on it it creates current. So you are basically pumping current into your motherboard. I haven heard that someone fried board like this. But i haven't researched the subject (which i should probably do). I know the physics and i don't mess with the laws of nature. And i recommend to people just to avoid it.
I told you to jam them if you are using air compressor. Those things can do serious damage to fans. Regular household vacuum cleaner isn't that dangerous.