The person who forgot an important detail may already understand that it mattered.

Reminding somebody of the weight of what they missed is nearly always redundant. Whoever forgot has usually spent the last hour running through the consequences, and an explanation of why it mattered arrives at a person who got there well ahead of you. What they need next is the recovery.

When someone drops something important, skip the part where you establish that it was important. Go straight to what happens now, and leave the lesson for a calmer day, if it is needed at all.