"An eephus ain't nothing. And that's what that pitch is...nothing." - Maurice Van Robays, who is said to have given the eephus pitch its name
The eephus pitch, or "blooper pitch," was a trick pitch thrown by Rip Sewell in the 1930s and 1940s. After an injury, Sewell learned to throw a slow pitch that had a high arc. It then dropped through the strike zone for a strike. The pitch was successful because it was so radically different than the pitches players would normally see. The only player to hit a home run off Rip Sewell's eephus pitch was Ted Williams in the 1946 All-Star Game. A picture later revealed that Williams was in fact out of the batter's box when he hit it and so it was therefore not truly a home run.
Other practitioners have included Steve Hamilton (who called it the Folly Floater), Bill Lee (who called it the "Space Ball" or sometimes the "Leephus"), Dave LaRoche (whose offering was called La Lob), and several others.