Ah ha!
Didn't read the known issues in the patch notes did you?
~evil grin~
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Known Bliz / nUI compatibility issues...
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- Right clicking a unit frame and selecting "set focus" taints in combat == this is a known bug since WoW 3.0.2 and is beyond nUI's control. Hopefully Bliz will address it in the next patch (or two or three or twenty). You can target the desired focus and then type '/focus' to aquire a focus frame until Bliz resolves this issue.