Unbilled encounters, missing procedure charges on a billed visit, uncaptured supplies and devices, hospital rounding never entered, under-levelled E/M, contractual underpayment, unworked denials past appeal deadline, timely filing lapses, abandoned patient balances, unreconciled credit balances, uncollected point-of-service co-pays, and unmanaged write-offs.
Contractual underpayment deserves particular attention: without loaded fee schedules in your system, you cannot know whether a payment was correct, which means the error class is undetectable rather than rare.