I have found a couple of old Zenit lenses in a drawer. I have found online, for a very cheap amount (about 10 eur) an adapter ring (from M42 to EOS mount).
The lenses have not been particularly well preserved in the years, so I don't expect to have found a treasure, but given the very small amount of money involved I could give it a go.
The question is: will I be risking anything using this adapter? Is there some specific thing to avoid or to do? If there is even a small chance of - who knows - damaging/bumping/touching something on the camera (specifically I have a 50D), I don't think that it would be worth it.
If on the the other hand the worse which can happen is that the lens turns out to be not really usable or that focus at infinity is unattainable, I will try it.