I'm at Varrock to train magic, I'm using curse on this locked down monk of zamarok, with -33 magic stats.
Usually, these attacks wouldnt hit on him and you'd see that failed magic splash instead, but with curse that isn't happening, it acts as if he got actually cursed.
So it looks like I actualy cursed them when I didn't, since I can still re-Curse them afterwards.
Summary, The curses fail, but the animation shows as if the curse actualy succeeded, when it should show the "splash" animation (failed)