When we leave our geared bikes in the parking lot, some curious guys may shift gears. To avoid damage, what I do is to leave it in the lowest speed gear combination, i.e., biggest chain-ring on the cassette (freewheel) side and smallest on the crankset. On a locked bike, shifting gears in this position will only loosen the gear cables.
For example, suppose, if I leave it at gear #4(of 7) and I try to shift to #1, there will be resistance after #2 and if I force further, damage is sure. This is because the cable is being pulled and tightened. On the other hand, shifting gears towards #7 only releases the cable and tension is created only by the spring of derailleur. So leave it at #1 when parking.
Just a theory. Additions and corrections to the above is welcome. I am still an amateur.