Apparently on March 4, 1981, he fought a guy named Miguel Ruiz and they recorded it as a disqualification at first but then changed it to a win later on. Here's a quote about it:
"For many years all the leading record books, including both Pugilato and The Ring Record Book, recorded the first loss of Julio Cesar Chavez as being by disqualification....
This has since been changed to a knockout victory for Chavez, based on confirmation from the local boxing commission in Culiacan that it altered the verdict the following day.
Ramon Felix, manager of Chavez, happened to be a member of the Culiacan commission at the time."
The A-Z of World Boxing by Bert Blewett (1996) at p. 341.