Explanation:

A credit purchase of machinery mistakenly recorded in the Purchase Book instead of the Journal Proper typically represents:

- Option 1: Compensating errors - Where one error offsets another, which isn't applicable here.

- Option 2: Errors of commission - Involves mistakes in recording amounts, entries, or from a procedural perspective, different accounts, while using the correct accounts’ nature.

- Option 3: Errors of principle - Correct here, as machinery is a capital item and should not be recorded as a purchase, which is for goods sold. It's an error of categorizing nature.

- Option 4: Errors of omission - Skipped or unaffected transactions, which is not the case here.

Option 3 (Errors of principle) is the correct choice