I use mouse over macros, and this happens to me all the time. It's very frustrating on my WHM. (Less painful on SCH because I have Embrace macrod to my Physick and Adlos, so fairy usually heals them while I'm here wasting a cure on myself.)

Face target does help a lot though, so you can quickly cancel and recast. Like if you're healing the tank, and didn't face towards them, that's a red flag. Lol