Have you changed your shifter cables recently?
Shimano and Campy riders should change rear shifter cables every three months under heavy use (200+ miles per week), every six months under light use. You may get longer out of your cables, but if you want to limit the risk of cable failure during a race or event frequent changes are recommended.
Due to the tight circle the cable follows around the ratcheting mechanism inside the shifter and the sharp, metal edges of the cable channel, Shimano and Campy shifters fray cables right below the cable head. (Sram uses a resin cable channel on most shifters and cables are much less prone to failure.)
Photo from http://binstedscyclingandtravelblog.wordpress.com/
The cables eventually break and can become jammed in the shifter internals, often causing irreparable damage. (We’ve got some special techniques to service supposedly non-serviceable Shimano shifters at DC, but they don’t work 100% of the time. Shimano will usually not warranty damage from broken cables.)
As riders only shift the front derailleur a fraction as often as they shift the rear, front cables generally last longer. Shops in Florida and other dead-flat states hardly see broken shifter cables at all due to how seldom riders shift.
In order to minimize damage from a frayed cable, if your shifting begins to work erratically, do not down-shift as this will only pull the frayed cable deeper into the mechanism. Try up-shifting—if your bike won’t up-shift, or if it labors to up-shift you may have a frayed cable. Leave your bike in one gear, pedal home easy, and change your cables!