When a GUI is open, the crosshair renders behind it. It is of course hidden in most situations, but it is aesthetically problematic in cases where it is not hidden.

When using the default resource pack, this affects the sign, command block and structure block GUIs, but it only looks particularly bad for signs (both hanging and regular) due to the way it partially overlaps.

When using a resource pack that renders GUIs completely or partially transparent however, including those that simply reshape the borders, (such as to visually separate the container/workspace from the player inventory) this can affect any GUI, and can look extremely ugly, and even confusing if not intentionally worked around.