The reunited New York Dolls jumped the shark with their "Cuz I Sez So" album. The album gets off to a good start, but then it takes a really weird left turn. "Dancing Backwards in High Heels" was also disappointing. I saw them live for the last time in June 2010, and the set list sucked and the sound was terrible that night (although that has to be blamed on the venue).
The Pipettes jumped the shark when Rosay and Riot Becki left the band, and Gwenno decided to keep it going with a more contemporary dance sound.
On a related note, sometimes I get angry about favorite bands breaking up or going on hiatus for all of eternity, but on the bright side, at least they called it quits before they disappointed me. At least I will always have happy memories of Manda and the Marbles and the Takeover UK.