HeroicFantasyGames.com

I started off a fight by casting Mind Blank on a character - only to have that character become sleeping due to an Arrow of Sleep.

It appears that the immunity check is made in the code for each spell/attack, rather than in the "apply condition" code (e.g., "apply sleeping").

You're right, with Sleep Arrows the game fails to check for an immunity to mind effects. Thanks for the bug report!