From a recent report by Social Security's Office of Inspector General (OIG):
Our objective was to determine whether the Social Security Administration (SSA) correctly completed manual actions to bill for Medicare premiums owed by beneficiaries whose monthly Social Security benefit was less than the monthly Medicare premium.
When an individual entitled to Medicare Part B receives a monthly Social Security benefit, SSA deducts the monthly Medicare premium from the benefit . However, some individuals’ monthly Social Security benefit is lower than the monthly Medicare premium. SSA must bill these individuals for the remaining amount of the Medicare premium. ...
SSA incorrectly calculated the Medicare premium owed for 120 (48 percent ) of the beneficiaries we reviewed who had a monthly Social Security benefit lower than their monthly Medicare premium. Based on these results, we project 33,092 beneficiaries paid incorrect amounts totaling almost $21.9 million for Medicare premiums because of SSA’s errors. These errors included miscalculations, erroneous system inputs, and failure to update beneficiary records correctly. As a result, some beneficiaries paid more than the correct amount for Medicare premiums, while others paid less ....