Don't call them mistakes. Call them opportunities to learn. We want all mentors to do a good job, and it is essential that they follow-through on their commitments. Mentors want to do a good job. But doing everything with excellence is humanly impossible, and attempting that results in burnout.
Every mentor has many roles.
Each has other commitments. You can't achieve excellence in every area. Desiring excellence in every area can become a trap - an impossible and unnecessary goal. Sometimes an adequate job is good enough. For some people, being adequate is equivalent to failure.