Jon Danzig, a former volunteer at the foundation, asked the regulator to investigate the charity after he used the Data Protection Act to obtain copies of derogatory emails sent by staff and trustees of the organisation to its members.
One referred to Danzig as a "dreadfully sad gentleman" and another urged recipients not to communicate with him.
The commission decided not to open an investigation into the foundation in August, saying that the dispute about the emails was a personal matter.
The commission wrote to the foundation, saying it required staff to mediate with Danzig about his complaint. But when the charity questioned whether mediation was compulsory, the commission admitted it did not have the power to force it to do so. According to Danzig, the charity has not contacted him.
A review panel set up by the commission to assess its handling of the complaint said in an email to Danzig: "The panel acknowledged that there had been some serious deficiencies in the standard of service that the commission had provided.
"It agreed that some of the correspondence from the commission had contained errors which were unacceptable. More specifically, the panel acknowledged that the commission had initially told the charity that mediation with you was a requirement, but that this had been subsequently changed to a recommendation."
The email offered Danzig £100 "in recognition of the embarrassment that the commission’s failure caused you".
Danzig told Third Sector he would donate the £100 to charity. "The commission’s intervention actually harmed my case," he said. "It sounded like they were backing down or had changed their mind about whether the charity had to mediate."
Third Sector was unable to contact the Pituitary Foundation for comment. The commission declined to respond.