I tried the spell again on some slept lowbie mobs (young cloud giant, Happy Farmer, a farmer). Out of the 40 times I cast it, I cursed myself 4 times, and got no other messages.

I think the problem is the spell simply does not land on targets. I tried to dispel the mobs after 10+ casts of pwc on each, they were not cursed. Normal curse worked on them still, with no message about them already being cursed.