Can a debt collector threaten me with arrest over unpaid debt?

Consumer debt, on its own, carries zero risk of arrest. It's a civil matter, not a criminal one, and a collector who implies otherwise is making a false statement, which both federal law and California law specifically prohibit.

How specific the threat was tends to matter for building a case. A collector saying "there could be consequences" is vague enough to be almost meaningless. One saying "the sheriff will be there tomorrow if you don't pay" is a concrete, false claim. If you can, capture the exact wording used, that specificity is what makes the claim provable.