Eyewall Industries have developed simple but useful device they called the STOKBLOK. What is does is prevent an M4-style adjustable stock from extending further than a desired distance and so preventing accidentally extending is to far when under stress. It seems like a very good idea for anyone who is forced to store their AR-15 with the stock collapsed.
Brownells are selling the STOKBLOK for $10.
UPDATE: The STOKBLOK is also compatible with the new Colt Superstoc. Eyewall Industries asked me to mention that the STOKBLOK is patent pending.Related