What we look for first
Three questions, in order. Are the important pages indexable? Is their content present in the served HTML? Do they load fast enough not to lose the visitor?
A negative answer to any of the three makes everything else pointless. We have seen sites publish two years of content behind a blocking directive left in place after a staging run.
The costliest mistakes, by frequency
Content rendered only in JavaScript. Indexable in theory, very unevenly so in practice once a site passes a few thousand pages.
Pagination and filters left open to crawling. Tens of thousands of generated URLs, crawl budget spent for nothing, and deep pages never reached.
Redirect chains. Three hops before the final page, each costing time and diluting the signal.
Incomplete hreflang. A version that does not declare itself, and Google ignores the whole cluster.
Preventing regression
Fixing is not enough: without a guardrail, technical debt returns by the third deployment. We put a performance budget and critical tag checks into continuous integration. A deployment that breaks an SEO signal is blocked before it reaches production.