You won’t believe how easy it is to open a garage door from the outside unless you take precautions. Check out this video from Lifehacker: How to unlock your garage from the outside and how to prevent it.

The vulnerability lies in the quick release on the garage door opener itself (the thing that pulls the door up and down).  If you immobilize the quick release the exploit won’t work.  You could also lock the garage door itself, but that would prevent the garage door opener from working at all.