How does it fail?
The plastic deforms over time and eventually fails to provide sufficient contact force for proper heat transfer and high CPU temperatures that eventually get bad enough to have throttling.
Everyone I know who has owned a Core2 with stock HSF has run into this problem after 3-4 years. Some of my friends who own(ed) LGA1156 CPUs with stock HSF are also starting to have the exact same problem. On these forums, you also periodically see the same story - people complaining about high CPU temperatures after owning a LGA775/1156 CPU with stock HSF for 3+ years.
So it seems the stock HSF is engineered to become mechanically inadequate after 3-4 years from material stress. Conveniently enough, Intel's warranty is 3 years. People think their CPUs are dying but it is simply the HSF starting to loosen too much. Replace the HSF and the PC behaves good as new.