Some lenses made by Ricoh have a pin that gets the lens stuck on your camera. They are not all that common, but it has happened. Check out this thread.
the dreaded ricoh pin
Other than that ANY lens that fits on either a K-mount or M42 screw mount adapter you can safely use on your K-r. Obviously you will have to get the M42 screw mount adaptor, but they are relatively inexpensive. Get the genuine Pentax one, some of the cheap knock offs don't allow you to focus to infinity.
NaCl(with all the K-mount and M42 mount lenses out there you have literally thousands of choices)H2O