I've had many M42 lenses of different brands and never had this problem on my K100D or K200D. Some Googling revealed that it's a known problem with some Yashinon lenses.
In
this thread on Manual Focus Forum there's even a quote from some old Yashica body manual:
"Because of the special construction of the mirror, the following Yashinon interchangeable lenses cannot be mounted on the yashica Electro AX:
Yashinon-DX 21mm f/3.3
Auto Yashinon-DX 28mm f/2.8
Auto Yashinon-DX 50mm f/2, f/1.7, f/1.4"
Apparently the "special construction of the mirror" is common in many other cameras including your K20D and the mirror was somewhat different in earlier Yashica bodies for which these lenses were originally meant for