The 1st boss in the Vault of Stars targets a flower with a green line and then shoots a projectile that buffs it. More importantly, during the 2nd phase he targets a pinned down player with a black line.
1. Even when the black line is visually blocked by the tank, the projectile can still reach its original target. This has happened to me only once and I don't have a video of it. But there was plenty of time to block, I didn't move after intercepting the line, the Gardener was not on significantly higher ground and I wasn't very close to him.
2. When the line is intercepted, it visually connects to the interceptor, significantly deviating from the projectile path.
For example, the green line can be intercepted by another flower, but the projectile shoots ~30 degrees off into a flower further away.
The black line can also be accidentally intercepted by the players (and maybe companions?) near the dog, making it jump around, which is very confusing for the tank.
When the Gardener is on elevated ground, these issues seem to be worse.
3. Sometimes the projectile bypasses the paladin's shield (blocking with Divine Palisade on) and hits HP without depleting the shield. This might have something to do with server lags or elevation.
4. Aside from the line issues, timing on the dog's jumps varies a lot. Sometimes there is plenty of time to notice the Gardener and prepare for the jump, other times the dog jumps 2-3 times back to back, sometimes without audio cue. It can also jump on the tank. I'm not sure whether that's intended.
Either do not move the line at all when it is intercepted (and indicate the interception by other means), or add an additional stripe on the ground, indicating the projectile path (similar to the red stripe that he targets players with, or the stripes in the Temple of Spider).
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Also when the Gardener puts arrows over everyone's heads, he sometimes switches target for 1 attack (possibly to a lowest aggro target), similar to Morzach in TIC switching after Flurry.
I can add some additional info on the line bugs:
If I were to take a guess, it's as if the visual for the lines and the actual paths of the lines are disconnected somewhere in the code. Whatever decision process is made on where to target is happening separately for the visuals and the actual power, resulting in the difference.
I have once managed to intercept the lines damage after the cat was killed mid-pounce by jumping in the path of the power after it was released (it has a travel time so this is feasible). It seems if the cat is killed during a pounce, the line resets.