Layout is no longer the problem
Most sites reflow acceptably. The failures that cost conversions now are interaction failures — things that work with a mouse and a large screen and quietly don’t with a thumb on a phone in daylight.
1. Hover-dependent behaviour
Anything revealed on hover is either unreachable or triggers awkwardly on touch. Dropdown navigation is the usual offender, which is one more reason important pages need a real link somewhere visible.
2. Tap targets too small or too close
Controls sized for a cursor produce mis-taps, and mis-taps in a form produce abandonment. This is where you lose them, and it is invisible in a desktop review.
Nobody reports that they mis-tapped your form and gave up. They just don’t appear in your inbox.
3. Text that assumes a wide column
Long words, wide tables, and code blocks force horizontal scroll or overflow the viewport. Wide content needs its own scroll container rather than being allowed to stretch the page, or the whole layout breaks sideways.
4. Hiding content instead of prioritising it
Collapsing sections on small screens is fine; deleting them is not. Content hidden on mobile is content most of your audience never sees, and if it was persuading people, you removed the persuasion for the majority.
5. Fixed elements eating the screen
Sticky headers, cookie banners, and chat widgets each take a share of a small viewport, and together they can leave almost no room for content. Count their combined height on a phone before shipping.
Test on a real device, once
Browser emulation misses touch behaviour, real network conditions, and how the page feels in the hand — much the same gap as why a site is slower for real users than in your tests.