When was the first scandal that tarnished Major League Baseball?

The first scandal that tarnished Major League Baseball was in 1878.

It was determined that four players on the Louisville Grays, George Hall, Bill Craver, Al Nichols, and Jim Devlin, had been paid off to throw games during play against Boston for the pennant; claiming they did so because owners were not paying their salaries. These players were banned from baseball for life by William Hulbert.