Certain (x, y, zoom) combinations will expose gaps between the world's map tiles on a monitor display (example here). Observed where zoom=5 and zoom=4, and likely occurs at other zoom levels too.
We have looked at this issue and its a problem with how we simplify the map geometry after generating it, which is sometimes a slightly lossy simplification causing small artefacts. Because this is a minor cosmetic issue, and because this simplification is important to performance, it is considered beyond the scope.