The reason I screwed up the current(?) elusive target is that I try to do it as fast as possible. And the fastest method I discovered required picking up the book first. Which means once I do that, I'm committed, because picking up the book is completing an objective. I did a lot of dry runs where I'd briefly pause at the book, but not pick it up, then execute my assassination attempt but miss on purpose, or execute the ET's guard instead to see if I could get away with it. The assassination method I decided on carried risk of being spotted, but usually I was able to do it (approximately) without being spotted in my dry runs. I got unlucky or was sloppy on my execution on my real attempt.
I think I could have immediately paused, unplugged my ethernet cable and restarted my computer to try again. But I just decided not to. You only need 5 SA ETs to get all the suits so I let my screw-up stand.
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When people say something to the effect of, "yawn, this elusive was so easy," I kind of laugh to myself and think, "yes, but you're doing it the boring way, mate."