42 C.F.R. § 417.458
An HMO or CMP agrees not to recoup deductible and coinsurance amounts for which Medicare enrollees were liable in a previous contract period except in the following circumstances:
(a) The HMO or CMP failed to collect the deductible and coinsurance amounts during the contract period in which they were due because of—
[50 FR 1346, Jan. 10, 1985, as amended at 58 FR 38082, July 15, 1993; 60 FR 45678, Sept. 1, 1995]