On most raids (I haven't seen this in a 5 man just raids.) There's usually one or two people that I can't target via grid or cast on via clique. If I was to change the grid layout, those people would be targetable but perhaps 1-2 OTHERS wouldn't be. I can redo the layout when I'm not in combat and that doesn't change anything so it's probably not an incombat dynamic frame type thing.
Thanks I'll try that tonight. I tend to think that's not the case since which unit seems to change each time I change the layout, however it is a possibility. There's a 'disabled' bartender frame that can be crossing where Grid is, and it's VERY close to what MetaHUD displays.