Speakers frequently make subject-verb number agreement errors in the presence of a number-mismatching local noun (e.g., *The key to the rusty old cabinets are on the table). A series of two-choice response time (RT) experiments was used to test an account of these errors according to which a number attractor generally makes the speaker's representation of subject number less definitive, with errors arising probabilistically from a competitive decision process. As predicted by this account, the presence of a number attractor reliably slowed responding when the correct response was issued. Analysis of RT distributions showed that this slowing was not due to pronounced difficulty on a minority of trials, but instead was manifested on most trials. Error responses were not slowed compared to correct responses, suggesting that errors and correct responses emerged from a single decision process. The data patterns were modeled using the Ratcliff diffusion model (Ratcliff, 1978), which explicitly assumes that variability in response output is due to random trial-to-trial variability in a range of decision parameters. Exceptions to these data patterns were observed in the case of non-intervening attraction, suggesting that this phenomenon may have a distinct cause. The results are taken to argue against standard accounts of number attraction, according to which errors occur in specific instances in which the speaker's representation of subject number is defective.