"sour note"

I'm looking for a formal definition for sour note. I can find sour grapes but it was not used in that context.

A newspaper article said "the coaches 2 decade career ends on a sour note". This was high school basketball and they lost the coaches last game. No one in the article had sour anything and the coach had great career. I believe the term "sour note" was not used in the right context.
Thanks

A sour note is any unpleasant event. For the coach, losing the last game would qualify.