Conn. Gen. Stat. § 10-183ff

(b) If a member or beneficiary has been overpaid through no fault of his or her own, and the member or beneficiary could not reasonably have been expected to detect the error, the board may waive any repayment which it determines would cause hardship.

(c) Upon determination by the board that any person has erroneously been included in membership in the teachers' retirement system, contributions and interest credited under the provisions of this chapter shall be refunded and records of related service voided.

(d) Upon determination that the board has invoiced a member for the purchase of additional credited service in an amount in excess of that permitted by law, and such member has paid the invoiced amount, the amount of the overpayment shall be refunded to such member with interest at a rate equal to the average of interest rates for the most recent ten-year period from the date of the member's retirement to the date such amount is refunded.

(e) Upon determination by the board that a member received, on or after November 1, 2008, an estimate of benefits statement from the board that contained a material error, the board shall pay the member the benefits set forth in such estimate if the board determines that (1) the member could not reasonably have been expected to detect such error, and (2) the member, in reliance upon such estimate, irrevocably submitted (A) his or her resignation to the employing board of education, and (B) a formal application of retirement to the board. For purposes of this subsection, “material error” means an error that amounts to a difference of ten per cent or greater between the estimated retirement benefits and the actual retirement benefits to which such member would otherwise be entitled.