A card in the projects grid that spans the full width and does not open a project is an advertisement. One is inserted after every five complete rows of projects. It is not a project card that failed to load, and it is not a fault in your browser.
The card is not targeted at you. Nothing about what you have viewed, saved or published decides which one you get: one is picked at random from the small set currently running, so two people scrolling the same feed see different cards in the same slot. Selecting one opens the advertiser's site in a new tab, leaving your place in the feed intact.
Why it moves around between screens
The gap is counted in rows, not in cards, and how many cards fit a row depends on the width of your window. A wide window fits more projects into five rows than a narrow one, so the same advertisement lands further down the feed. Resizing the window recalculates the position live, which is why one can appear to jump while you are dragging a window edge.
They are never placed at the very end of what has loaded, so the last thing you scroll to is always a project.
Where else they appear
The same card appears in Projects search results, and one sits in the right hand column of a project page. Journals, publications and competitions carry their own.
There is no ad free setting
An active membership does not remove them, and there is no preference to turn them off. If a particular advertisement is misleading or inappropriate, write to support with the name on the card so it can be looked at.
If the slot is blank or grey
An empty box in that position means the request for an advertisement did not return one. It is cosmetic, and reloading the page usually fills or removes it.