Given the large contribution of human error in the failure of complex systems, understanding the source of errors is an important issue. It has been proposed that, in speeded situations, responses biases induce subjects to guess which response will be required. When the guess turns out to be wrong, a fast guess error occurs. In unbiased conditions the possible contribution of fast guess errors remains an open question. We used a response-locked event-related potential (N-40), assumed to reveal the presence of a response selection process during the reaction time, to probe the presence of a response selection in biased and unbiased situations. The N-40 was present without response bias but absent in biased situations. This lends physiological support to the idea that, in a priming paradigm as used here, most errors in biased conditions are fast guesses whereas most errors result from inappropriate response selections in unbiased conditions. This reveals different sources of errors.