I have a few scripts that recently broke because (I think?) of this change: https://gerrit.wikimedia.org/r/c/oojs/ui/+/748066

They were doing thing.addItems( item ), rather than thing.addItems( [ item ] ) and I have corrected them, after tracing the breakage.

While the change linked above is perfectly valid, it's possible more scripts may break here if they have accidentally been using a wrong, but previously working, call without the square brackets. Whether this ever worked is probably up to the widget in question (in my case: FieldsetLayout and ButtonSelectWidget). However lots of JS APIs have an item-or-array style, so it doesn't look obviously wrong to do so, and the type annotation in the docs said items, not [items], so it might have both worked and looked plausible. So this issue can be a place to hang such reports.

For example, a quick trawl though Wikipedia finds a few things like this:
https://en.wikipedia.org/wiki/User:Terasail/COI_Request_Tool.js

        var dropMenu = new OO.ui.DropdownWidget();
        ...
	for (var count = 2; count < replyListC.length; count++) {
		var newOption = new OO.ui.MenuOptionWidget({
			...
		});
		dropMenu.menu.addItems(newOption);
	}