When RPM drops after a traffic spike, what do you check first?

A creator analytics question about separating traffic mix from ad layout problems.

A traffic spike can make RPM look worse even when the site is not technically broken. I would compare source, country, device, landing page, and session behavior before moving ad units everywhere. Otherwise one social or discovery-feed spike can push the site average down and make normal evergreen pages look worse than they are. When you see RPM fall after a spike, what is your first check: source mix, page layout, ad fill, or something else?