A very large possiblity I would say. If the futures are a leveraged bet, someone heavily short the futures and holding large amounts of BTC would have a large incentive to sell the btc hard to win on the contract and then buy back the BTC lower. With large selling comes stop runs and panic selling so thier selling facilitates much greater selling at certain price points. With computers and algorithyms available to only rich investors, this becomes a pretty doable arbitrage.
Key for small players is, hold your bitcoin through contract expiration dates.