You would have contact lenses in the wrong eyes.
Now seriously. I'm guessing you mean what happens if you accidentally put them in the wrong eyes. There would be no damage or pain (providing they have been disinfected correctly and that you don't always wear them the wrong way round) - they are made of the same material and are not chemically made specifically to each eye! Only the optical strength is different.
That's like asking what would happen if you did this with your shoes. You'd just feel weird and walk funny.
You would know by the fact that your eyesight doesn't seem quite right, compared to when you have put them in correct way.
Contact lens wearer.