The mistake is rarely what damages the relationship. What damages it is the twenty-four hours afterwards.
What happened, which accounts, what the financial effect is, and when it started. Before contacting anyone.
A partial explanation that changes twice is worse than a complete one an hour later.
This is the whole thing. A client told about an error by their adviser has an adviser who is watching carefully. A client who discovers it themselves has an adviser who missed it and might have concealed it.
Same error, entirely different conclusion about you.
Clients forgive errors far more readily than they forgive finding out on their own.
What happened, what it cost them, what you are doing about it, and what prevents a recurrence.
Do not lead with the explanation of how it happened. That reads as excuse-making, however accurate it is.
If the error cost them money, correcting it is not generosity. Consult your compliance consultant and your insurer on process, but the instinct to make it right quickly is correct.
What happened, when you found it, what you did, what you told the client. This goes in the file. A well-documented error handled properly is a much smaller problem than a poorly documented one handled well.
Almost every error in a small firm is a process that permitted it rather than an individual who failed. If the response is to be more careful, it will happen again.
Staff report errors early in firms where reporting early is safe. In firms where it is not, errors surface later and larger.
How the principal reacts to the first mistake sets that for years.