Ah yes, semantics, error, anomaly, variety!
The following is my personal view, and my personal view only, as it relates to our hobby.
An "error" is something that does not have "predictability" associated with it. Missing one of the printing steps, printing registration shifts, printing on the wrong side, paper folds, etcetera, etcetera, are errors.
An "anomaly" or "variety" has predictability associated with it. The missing back plate position number on the Bird twenty is a variety produced by a plate anomaly. Predictably, every sheet printed from this plate had the condition. The missing circle note is a variety produced by a plate anomaly which predictably made 25% of the notes potentially missing circle notes.
JWS