I can think of many races where a lot of the top favorites dropped out or couldn't even get to the start line.
Tour de France 2006: Ivan Basso and Jan Ullrich get caught up in Operacion Puerto and were banned. Valverde crashes out.
Tour de France 2008: defending champion Alberto Contador is not invited, just like third place finisher Levi Leipheimer. Michael Rasmussen was banned. So that's the top 3 of 2007 that couldn't contest for the win in 2008. Who won the Tour that year? The guy who finished fourth in 2007.
Tour de France 2012: Alberto Contador is banned, Andy Schleck crashed in the Dauphiné and couldn't start the Tour that year. Wiggo's strongest rival was his own team-mate, who was kept on a leash. It was also a Tour with many time trials and barely any decent mountain stages. This has got to be one of the most luckiest Tour wins ever.