It's not super-common, but you do see sometimes sprinters and points jersey competitors getting into mountain stage breakaways in order to have a bit of a headstart on the time cut - especially with a gradual first climb that the sprinters feel they can get over unscathed. That said, Sagan shouldn't need to worry about the timecut, he's not exactly a Van Hummel.
A couple that I recall quite vividly along with Thor's exploits in 2009-10 are Alessandro Petacchi in the queen stage of the 2004 Giro, going over Tonale with the break and being dropped on the Gavia, and Óscar Freire in 2008 - I think it was the Jausiers stage, which would have meant he went over the Col de la Lombarde in the chasing bunch (Stefan Schumacher was off the front solo) and was dropped on the Bonette. Other times I recall sprinters in breaks are Danilo Hondo to Peio Terme in the 2010 Giro - he was 2nd on the day - a couple of moves with Bozic in the 2009 Vuelta, Freire's solo escapade in the 2012 Amstel Gold Race, and of course the most famous one of all, Djamolidine Abdoujaparov winning a medium mountain stage in the Massif Central attacking his breakmates on the final climb to the finish.