The Symphony No. 3 in E-flat major, K. 18 by Wolfgang Amadeus Mozart is, by today’s scholarship, considered to be not Mozart’s own work but instead that of Carl Friedrich Abel, a leading German composer of the earlier Classical period.
It was misattributed to Mozart for this reason: A manuscript symphony in the hand of Wolfgang Amadeus Mozart, was categorized as his Symphony no. 3 in E flat, K. 18, and was published as such in the first complete edition of Mozart’s works by Breitkopf & Hartel. Later, it was discovered that this symphony was actually the work of Abel, copied by the boy Mozart – evidently for study purposes – while he was visiting London in 1764. That symphony was originally published as the concluding work in Abel’s Six Symphonies, Op. 7.