It is in my opinion that, next to Blink, Shadow Kill is the most useful and timesaving ability in the game. The only problem is, IT DOESN'T WORK. The only thing it seems to do is make your targets invisible TO YOU!
Bug 1. Phantom moaning. You kill someone. He turns to ash. You can sometimes hear him moaning, gasping, etc, for a few seconds after- wait what?! Is that the sound of his ghost departing for the afterlife?!! Nope. Bug. This usually happens when you stab someone up the jaw from the front.
Bug 2. Flame without ashes. The incendiary bolt does not usually cause people to turn to ash after they're dead, even if they haven't seen you, which is ironic since you'd expect setting something on fire to result in ashes galore. However, to my surprise, the incendiary bolt CAN turn people to ashes but only if your targets HAVE seen you, the exact opposite of normal Shadow Kill conditions. I discovered this when I set a bunch of weepers who were coming after me on fire in "Lady Boyle's Last Party". I noticed a bunch of weepers were turning to ash even though they were clearly chasing after me.
Note: Taking more than one shot to kill your target with the crossbow bolt or pistol also prevents them from turning to ash even if they haven't seen you. Also, sometimes enemies that you;ve killed when time is stopped also don't turn to ash, which is odd since they can't see you when you're BEHIND them, much less when time is stopped. "Little" details matter.
Bug 3. (Dealbreaker) Bodies aren't really gone. This is the biggie. You kill someone. He turns to ash. You hide and observe out of sight. Then, a few seconds later, someone walks up to the place where there should be a body, but there ISN'T, cuz' y'know, IT TURNED TO ASH, and OUT OF NOWHERE starts saying things like, "Alright, where are you!? I know you're here!" Or this one time, in the Distillery District, I killed a thug (he turned to ash), and a few seconds later another thug walked up to that exact spot and went, "A DEAD BODY?!! Hmm something's goin' on here. Suspicious.".
I can reproduce this bug fairly easily right at the beginning of "The Royal Physician". There are four guards patrolling. One's in the booth, another walks around the area of the door leading to the next area (there's a dumpster right next to the door). The third guard is usually leaning against a wall directly in the spotlight, and the final guard walks the most, up and down the long road.
First, I take care of the guard in the booth and then the guard who likes to stare at the view by the edge. Reproducing the bug takes some opportunistic timing. You have to kill the guard leaning against the wall without alerting the other one. Also make sure the place where the body should be is in plain sight. When the other guard walks by, he'll "see" the body (sometimes he'll literally say, "SH1T! A DEAD BODY!" when there clearly isn't any body). I can post a video if people really have problems reproducing this.
That's just one example though, there are many, many others. Like with enemies leaning over a high ledge, even when they've turned to ash you can still hear their bodies crashing to the ground, and so does everyone else, alerting them to your presence. Grrr.
I suspect this bug is also why my stats at the end says that dead or unconcious bodies were found even though I was obsessive in making sure that's impossible. Very frustrating.