A cross controlled stall can result in a spin (which is probably what you mean by flip upside down). The rudder changes aren't inadvertent, they're intentionally opposite the aileron input - the goal is essentially to fly somewhat sideways, so the fuselage induces drag.

In general forward slips are safe, but yes you have to make sure you keep the nose down/speed up. There's little in aviation that isn't dangerous if you aren't careful.