What still trips sites up in mobile-first indexing
Mobile-first indexing is done. Google finished the transition back in 2023, and Googlebot Smartphone is the crawler that matters for essentially every site. Your mobile page is your page; the desktop version is, from an indexing perspective, a nice-to-have.
Yet the same handful of mobile-first mistakes keeps showing up in audits, quietly costing sites content and rankings. Here they are.
Content parity, still the big one
If content exists on desktop but not on mobile, it does not exist. That is the entire rule, and it is still the most common violation. Designers trim "secondary" copy on small screens, developers remove it from the mobile DOM, and a chunk of the page silently leaves the index.
The nuance people miss: content hidden behind accordions or tabs on mobile is fine, as long as it is present in the HTML. Collapsed is indexed. Removed is gone. Check the rendered mobile DOM, not the visual design, before signing anything off. Structured data and meta tags follow the same rule: mobile version or nowhere.
Lazy-loading that hides content from Googlebot
Mobile templates lean harder on lazy-loading, and that is where things break. Googlebot does not scroll or click, so anything that loads only on user interaction (scroll-triggered products, "load more" buttons, infinite feeds) is invisible to it. Native loading="lazy" on images is safe; IntersectionObserver-based loading of primary content usually is not, unless it fires within the rendered viewport. The test is simple: inspect the rendered HTML in the URL Inspection tool, and if your third row of products is not in there, it is not in the index either.
Interstitials and overlays
Intrusive interstitials remain part of Google's page experience guidance, and mobile is where they do damage: app-install banners covering the content, newsletter modals firing on entry, consent implementations that push the actual page below a wall. A poorly built overlay can also dominate the rendered snapshot, so the crawler sees your modal instead of your content. Keep entry overlays small, dismissible, and out of the way; properly done cookie notices are exempt.
Mobile-only errors
Sites that serve different mobile experiences (dynamic serving, separate templates, aggressive device detection) can fail in mobile-only ways: the mobile template throwing errors while desktop is fine, robots directives that differ between versions, or bot-mitigation rules blocking Googlebot Smartphone specifically. The Crawl Stats report in Search Console breaks requests down by Googlebot type, and server logs show smartphone-crawler errors your desktop monitoring never catches. If mobile Googlebot gets 403s or 500s, index coverage decays no matter how healthy desktop looks.
Viewport and font sizing basics
Still worth thirty seconds: a correct viewport meta tag (width=device-width, initial-scale=1), body text at 16px or equivalent, and tap targets that are not stacked on top of each other. Google retired the standalone mobile-friendly test, but Lighthouse in Chrome DevTools and PageSpeed Insights still flag these issues, which also correlate strongly with the INP problems that surface later in CrUX data.
Test as the smartphone bot
Finally, verify what the actual crawler experiences. Set Chrome DevTools device emulation to a mobile viewport with the Googlebot Smartphone user agent string, or crawl the site in Screaming Frog with the preset Googlebot (Smartphone) UA and JavaScript rendering on. Compare word counts, links, and structured data between desktop and mobile crawls. Any significant delta is a parity gap, and in a mobile-first index a parity gap is a loss.
Mobile-first indexing stopped being news a long time ago. It did not stop being a checklist. Run it once a quarter and it stays boring, which is exactly what you want from your indexing.