The camera that was a real pain with M42 was my K-r. I wrote
this piece about it. Basically if there wasn't an electrical connection with the data contact on the camera mount (that's the 7th one at ~half-past) metering in Av was so out is was just about at the limit of the compensation (+3). Both my K5 and my K3-ii have been way better. These are some quick test results with a photax 35mm f3.5 prime (f3.5; f5.6; f8; f11), this is a typical M42 lens with a black anodised base and an A_M switch (and a sticky iris, but flicking the A-M switch takes care of that). Results are a bit inconsistent, tending to increasing overexposure, but not excessively so.
It's very difficult in actual use to distinguish this sort of variability from other metering issues due to patchy light, light/dark subjects, reflections, glare etc.
Not in order note the shutter speeds.