I think naked short selling of equities could lead to some problems - it's got to do with the value of the business versus the outstanding short interest. If we allow the latter to be unbounded, it's clear at some point it's in someone's interest to hurt the business. But this is mostly a theoretical argument so far - I can't think of any instance where this might have happened. It's the CDS - bonds problem, when the derivative vastly outnumbers the underlying, perverse situations can occur.