Do not call an error harmless until you know who had to absorb it.

Mistakes are usually assessed by the person who made them, from a position where the consequences are invisible. The wrong file, the missed message, the late change all look small from the inside, because the effort of absorbing them happened somewhere else: a weekend, a rushed apology, somebody’s plan quietly rearranged. Harmless is a verdict the affected party is better placed to give.

Before filing something under no big deal, find out what it cost downstream. Ask the person who dealt with it. Then adjust your sense of the mistake to match what you learn.