The broker usually are responsible if it can be proven and if a loss to a customer has been incurred. I remember a big fiasco in ELX few years back, someone spread a false rumor, the stock dropped from above $100 to $45 or even lower and finished above $100 (I got lucky to catch a ride, a daily double, not a double bucker, a doubling of the price, still sharply embedded in my memory). A lot of people blamed brokers for not "supporting the price or for erroneous low bids, none got compensated then, as far as memory serves. Lot od people got whipsawed.
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