Healing spells landing on me instead of my target

[Lilita]

1) Lag. The most obvious but the easiest to anticipate. Just make sure that the party member is highlighted before pressing any key
2) Target moved out of range - Usually only a problem in dungeons and large arenas. This is generally why I use the mouse to change healing targets, if you can't click, you can't target.
3) Target moved out of line-of-sight - This is probably more like what you experience. The best example of being able to recreate this is with a Scholar. Tell the pet to stay and then walk behind objects and keep casting Psysick on it until one lands on you.

When that happened to me, I had tank selected as my target for a minute or more before 1 heal randomly landed on me, I was standing next to the tank and after this happening so many times I also tried moving out of range of tank and recreate it - if i go out of range and cast a heal it simply says Out of range and my char doesn't cast anything (i do not use any macros).