Questions & Answers about Pero no es ningún error grave.
Why do we use ningún here instead of ninguno?
Why are there two negative words (no and ningún) in this sentence?
Spanish uses double negatives frequently! If a negative word like ningún comes after the verb (es), you are required to put no before the verb. It translates naturally to 'it is no serious mistake' in English.
Does the word ningún change the meaning compared to just saying un?