I bought a m960 rear derailleur, used it for about 10 rides max, on flat ground its never been hit, I keep it clean and the bottom wheel fell off when i was riding 20 miles out into the Everglades! when it fell off the chain grabbed the back of the derailleur and twisted it into a distorted ball of painful walking!!! 20miles in the hot ass sun of florida. well my point it, should this derailleur have broke like this? or was it defective and will shimono reimburse me? are they guaranteed for this?