Bhagavad Gita, Chapter 18, Verse 22

Whilst that which is confined to one single effect as if it were the whole, without reason, without foundation in truth, and trivial, that is declared to be Tâmasika.

Real confusion is named here plainly, mistaking one small piece for the entire real picture.

Krishna doesn't call this ignorance harmless, he ties it directly to a lack of real grounding in either reason or truth. Partial understanding treated as complete understanding is the actual problem.