The most common issue with Pentax ME Super, one of the most popular consumer-grade manual focus SLRs, was caused by a degraded rubber ring near the mirror assembly.
The ones without such an issue is still often recommended for hobbyists who want to get into analog photography today, because they are generally inexpensive, compatible with very nice lenses, and there are so many of them circulating still for parts and repair.
Given that they were made in the late 70s to early 80s, and the shutter mechanism is fully controlled by electronics (instead of mechanical), it's quite amazing that the rest of the camera mostly held up over nearly 5 decades.
https://www.ifixit.com/Guide/Pentax+ME+Super+Mirror+Box+Serv...
The ones without such an issue is still often recommended for hobbyists who want to get into analog photography today, because they are generally inexpensive, compatible with very nice lenses, and there are so many of them circulating still for parts and repair.
Given that they were made in the late 70s to early 80s, and the shutter mechanism is fully controlled by electronics (instead of mechanical), it's quite amazing that the rest of the camera mostly held up over nearly 5 decades.