The magic-user spell hallucinatory terrain
has a couple of oddities. Firstly, it states that a 1' square area per caster level can be affected. So, at the pinnacle of his mastery of the dark arts, a magic-user can hope to affect a 20' square area!
This seems to be a mistranslation from AD&D, where the spell affects a 1" square area per caster level (= a 10' square area, so a 200' square at 20th level, which is more like it!). Secondly, the duration is stated as "see below", but then not clearly described in the spell's text (it is supposed to be permanent unless touched or dispelled).