I had a similar issue when the patch dropped. Here are some macros to make sure your mistargeting doesn't result in you not casting on the boss:
Code:
#showtooltip
/cast [@target,harm,nodead][@focus,harm,nodead][@boss1,harm,nodead][@boss2,harm,nodead][@boss3,harm,nodead][nochanneling][] Smite
Code:
#showtooltip
/cast [@target,harm,nodead][@focus,harm,nodead][@boss1,harm,nodead][@boss2,harm,nodead][@boss3,harm,nodead][] Penance
Code:
#showtooltip
/cast [@target,harm,nodead][@focus,harm,nodead][@boss1,harm,nodead][@boss2,harm,nodead][@boss3,harm,nodead][] Light's Wrath
Also make sure to turn off auto self-casting in your WoW settings.