On an episode aired June 8 and 11, 1984, a self-described unemployed ice cream truck driver named Michael Larson made it onto the show. Watching the show at home and with the use of stop-motion on a VCR, Larson discovered that the presumed random patterns of the game board were not random and was able to memorize the sequences to help him stop the board where and when he wanted. On the single game in which he appeared, an initially tentative Larson spun a Whammy on his very first turn, but then played 45 consecutive spins without hitting a second one. He earned a total of $110,237 in cash and prizes, a record for the most money in cash and prizes won by a contestant in a single appearance on a daytime network game show. Although this record lasted until 2006 when Vickyann Chrobak-Sadowski won $147,517 in cash and prizes on the Season 35 premiere of The Price Is Right, it still remains the record for highest single-day winnings on a series with returning champions.
Although CBS investigated Larson, they determined that figuring out the patterns was not cheating and let him keep his winnings. The board was reprogrammed for more (and more complicated) patterns to prevent another player from being able to memorize the board like Larson had.
Michael Larson’s wiki entry goes into detail on how he actually did it.