Here’s something I see a lot (I’ve redacted the conference name because this is a teaching exercise, not a public shaming):
On a big viewport, those absolutely positioned floating widgets are tucked innocuously off to the side. On a small viewport, they completely obscure large areas of important content, making it unreadable.
Fluid grids aren’t enough. Test all of your content on as many devices at as many viewports as you can.
Note: This post should have been titled, “When unimportant content ruins important content,” but that’s less catchy.