If you play a song by either using a double mouse click or double keypress, it will show as if it were casting/singing, but the song never actually lands. It is also possible to see this behavior with /melody, but the second /melody activation has to be slightly more delayed.
Any updates on this issue? It is still happening and, while not game breaking, is extremely annoying. If it helps an investigation, this may be be related to code that causes a song to try to re-cast if interrupted/cancelled in the first fraction of a second after you begin casting. E.g., if you cancel a song immediately after it starts casting, it will begin casting again (makes interrupting melodies harder).