abstruse / obtuse
Two adjectives that are easy to swap and should not be.
Abstruse describes an idea, argument, or subject that is hard to understand. Obtuse describes a person who is slow to grasp a point, or an angle greater than 90 degrees and less than 180. A text is abstruse. A reader who misses the point is obtuse.
Examples
Use The commentary on the poem is abstruse.
Use He was being deliberately obtuse.
Avoid The commentary on the poem is obtuse.