The person affected by your error needs the repair before the account of how unusual it was.

After a mistake there is a strong pull to establish that it was out of character, and that pull serves the person who made it. The other party is still standing in the consequence, waiting to know whether it will be put right. Context offered first sounds like a defence, however true it is. Order matters more than content here.

Lead with what you will do and by when. Fix it, or say exactly how it gets fixed. If an explanation is wanted afterwards, it will be asked for, and it will land much better then.