Pimlico officials called in extra police officers to help quell a disturbance yesterday after the seventh race.
A large number of disgruntled fans gathered in the grandstand after they were shortchanged in a triple payoff.
The track paid out $33.30 on an erroneous 4-2-all combination instead of paying $172.80 on the correct 4-2-1 combination.
Steward Clinton Pitts Jr. said the error occurred because new Maryland Racing Commission rules allow for coupled entries in triple races. However, track management had not yet programmed the computers to reflect the rule change.
Track operator Joe De Francis said: "We had a lot of police here anyway [to prepare for the Preakness], and when there was confusion about the payout, there was a lot of screaming and yelling. The police just helped to control the crowd, but nothing got out of hand."