I had three 5600 105 rear shifters each wear out at about 15K miles. The internal detent/rachet was worn so it wouldn't catch on the 5th tooth. You could shift from 4th to 6th and from 6th to 4th, but there was nothing to hold 5th. A simple test is to pull on the cable as you downshift with the small lever. If it pulls twice as far on one of the downshifts, that's the one that's worn out. A new 9-speed Sora from 2013 (different bike) jammed on the thumb lever at 6K miles. Shimano replaced it with a NOS Tiagra 9-speed. It now has 14K miles on it. I'm starting to get nervous.