Gently let them know that they are wrong. Depending on the severity of the error, repeat the procedure up to three times with decreasing level of gentleness unt

Gently let them know that they are wrong. Depending on the severity of the error, repeat the procedure up to three times with decreasing level of gentleness until they become right. If that doesn't work, don't bother. Also isolate yourself from possible ramifications of the error.