I had noticed before that answers sometimes receive upvotes almost immediately, and had been wondering how the upvoter had been able to read and assess the answer so quickly, but so far there had always been at least something like $10$ seconds between the posting and the upvote.
However, today I posted this answer (now deleted), and I never got to see it without upvotes – by the time the page had loaded, the upvote counter already said "$1$". The page didn't take unusually long to load in my browser, so the upvoter must have loaded the page just when the answer appeared and then almost immediately clicked to upvote.
Now usually one might perhaps have thought that they had also been working on an answer and thus knew the answer, saw the correct result at the bottom and immediately clicked. That would still be somewhat negligent, since the rest of the answer might be wrong or misleading or badly written, but at least it would be acknowledging a correct result.
It just so happens, however, that the answer was completely wrong and didn't answer the question, since the question asked for the first moment and I (being a physicist) associated "moment of inertia" and calculated the second moment instead.
So this upvote was almost certainly not based on an appreciation of the answer; and that makes me think that some of those other surprisingly fast upvotes that I've seen in the past weren't, either.
Which raises the question: Who's doing this, and why, and how can we prevent it?