Safe - those activities which do not irreparably damage either the body or the mind. Blindfolded knifeplay is unsafe, banishment to a sub-zero basement as punishment is unsafe, agreeing to being hung by the wrists until your shoulders dislocate is unsafe, being left bereft of human contact as a way of guaranteeing fidelity is unsafe.
Anger makes any activity insane, so do drugs or alcohol.
Psychological imbalance is real hard to judge or fathom but activities with knowledge of an imbalance that would affect it is insane. Fear is just as devastating as anger when it comes to coloring judgement, therefore, activities entered into out of fear are insane. Again, YMMV, and I am recommitting these to heart. The copyright of certain works is held by the author or photographer where indicated.