Mistakes that cost the most time

Filing with a family register obtained before the marriage was registered, so the foreign spouse does not yet appear in it.

Writing the ceremony date where the form asks for the registration date. The two are often months apart and the family register shows only the latter.

Typing into the 自筆 (written in one's own hand) boxes.

Submitting a residence certificate that shows only one person when a household copy was needed to evidence cohabitation.

Unpaid residence tax discovered when the 納税証明書 (tax payment certificate) is issued. Clear it first; the certificate then shows no arrears.

Photographs that are all selfies from one afternoon. A spread across time, places and people is worth far more than volume.

Leaving item 24 blank on the assumption it is required. It is not -- this status has no work restriction, and no job arranged is a perfectly ordinary answer for an incoming spouse.