We show that bid-ask errors in transaction prices are the predominant source of apparent price reversals in the short run for NASDAQ firms. Once we extract measurement errors in prices caused by the bid-ask spread, we find little evidence of market overreaction. On the contrary, we find that security returns are positively, and not negatively, autocorrelated. We also show that bid-ask errors lead to substantial spurious volatility in transaction returns; about half of measured daily return variances can be induced by the bid-ask effect.