Norm Cash and Brady Anderson had respectable careers by major-league norms, but each also had a "career year" that stood far above his other accomplishments as a player.
In Cash's case, the one-season wonder was his league-leading batting average of .361 in 1961. It was Cash's first and last .300 season in a career that included 14 full seasons of play. His second-best average was .283; his second-best finish was in 1969, when his .279 average garnered seventh place in the AL batting race; and his career average was only .271.
Anderson's anomalous 1996 season saw him slug 50 home runs, finishing second in the AL to Mark McGwire. Anderson logged nine other full seasons of play, but in none of those seasons did he hit more than 24 home runs. He averaged only 19 home runs per 162 games over the span of his career.
Nevertheless, Cash and Anderson weren't true one-season wonders. That "accolade" should be reserved for the likes of Gene Bearden and Mark Fidrych. Both were pitchers who had outstanding rookie years -- Bearden with a W-L record of 20-7 in 1948; Fidrych with a W-L of 19-9 in 1976 -- and then faded quickly, departing from the big leagues after brief, mediocre careers. (Actually, Fidrych faded after July 20 of his rookie season, by which date his record was 11-1; he went 8-8 for the balance of the season.)
Among position players, there's Joe Charbonneau, AL Rookie of the Year in 1980 with a .289 average in 131 games. After that: .210 in 48 games, .214 in 22 games, and ... gone from the majors.
Bearden, Fidrych, and Charbonneau are among the true one-season wonders of baseball.
