To have a cup of coffee technically means a brief trial in the Major Leagues by a player or a pitcher. A rookie, often called up in September, will get their first taste of the major leagues and it will often be short. Figuratively speaking it's barely long enough for them to drink a cup of coffee.
Baseball Almanac has posted below a list of every former Major League player who had exactly one cup of coffee, or in terms of baseball statistics, played exactly one baseball game. Some pitched exactly one game (and never had an at bat), some batted in a game (and never fielded), some pinch ran (and never batted or fielded), but they all have had exactly one cup of coffee.
NOTE: The Cups of Coffee script is sortable! Click any column name (Only Game, At-Bats, Triples, Home Runs, Earned Run Average, Strikeouts) to see which cup of coffee player is the leader in that particular stat.
Baseball Players Who Appeared in Exactly One Major League Game