Off the top of my head:
2006: Strengthened a World Series win. Offense became damn near unstoppable. Rotation falls apart.
2015: This fucking offseason. Samardzija. Melky. Robertson. Bonifacio. LaRoche. Has there been a more devastating butterfly effect season in team history? After these additions failed (and Shark left) they added Frazier and filler (Rollins, Latos, and of course, Shields mid-season) to try and supplement the core of Sale, Quintana, Eaton, and Abreu. I remember rumblings that Hahn wanted to begin the rebuild after 2015, but apparently it took an extra year of catastrophe to convince the powers that be that no, this was not going to work.