Keep it

The mistake

The newsletter template uses a merge field like Hi {{FIRSTNAME}} that gets replaced with each recipient's name at send time, but the placeholder is left in the draft as literal text instead of a working merge tag, and no test send or preview step catches it before the campaign goes out. The send runs against the full 400,000-row customer list with the interpolation never firing, so every recipient gets the raw placeholder string instead of their name. There's no pre-send checklist or approval gate stopping a visually 'finished' draft from shipping to the entire list.