The mental illness of a drummer or percussionist. Causes the drummer to stare at a cowbell with desire to play it leading to a point where they play it at an inappropriate part in a song. People with MCD play the cowbell often as possible and it can get annoying after a while to some, but to most, it is entertaining.
"Did you hear John during practice today?"
"No what happened?"
"His Mad Cowbell Disease led to a corny drum fill and it threw everyone off. He needs to control himself."