Null handling
Defects in code that handles the absence of a value, where a null reaches an operation that assumes a live object. The failure is the familiar NullReferenceException, but the root cause is a contract about nullability that one side did not honor.
A representative case is calling an instance Equals(null) on a receiver that is itself null: the comparison throws before it can answer, where the static object.Equals or a null-tolerant pattern would have returned a clean result.