VW Jetta TDI (2005-2006) Camshaft failure:
While the root cause of these issues are still under discussion (see tdiclub.org for more information than you could read in an evening), the camshafts in the Volkswagen Pumpe-Duse (BRM) engines are prone to failure. Most of the time this occurs at higher mileage; over 100k miles. Not all of these diesel engines will experience the failure, but they occur with enough prevalence that it has become a widely-discussed problem.
Video of telltale “bupping” noise indicative of a failed camshaft:
I had two oil analyses performed during oil changes after noting the cam wear at 85k in my vehicle (for those who asked, I used Blackstone Labs. They have given me helpful feedback as well as the objective numbers on the reports). The BRM engine oil change interval (OCI) is recommended at 10k mi with 505.01-spec oil, which I initially went with. After seeing high iron content (below) I installed a magnetic oil drain plug and shortened the OCI to about 5-6k mi.