To say the obvious, telling the reader what will happen in a book can be a spoiler.
If anyone told me I could bring down the president, and the Pure Movement, and that incompetent little shit Margan LeBron in a week's time, I wouldn't believe them. (Vox, Dalcher)
Total spoiler. The chance of a spoiler ocurring on any random sentence is very small, but the first line is, of course, different.
The year that Bibi Blair turned ten, which was twelve years before Death came calling on her, the sky was a grim vault of sorrow nearly every day from... (Ashley Bell, Koontz)
Spoiler. Also, the reader knows better than to identify with a character who will die. Was that really the author's intention? Almost never. (Also, my job, as reader, is to construct a world. I have no idea what I'm supposed to do with that goofy piece.)
An Explanation of Spoiling
It seems to be well-accepted that learning the outcome, such as the resoution of a conflict, is a spoiler. That type of spoiling isn't hard to explain.
But suppose something surprising and amazing happens to a character. Can surprise be spoiled?
To make a long story short, one of the goals in writing is that, when the character is surprised, the reader will also be surprised. The basic technique for this is to give the reader the same information the character has.
So advance knowledge can spoil surprise. More generally, it can stop the reader from having any emotional response the character is having. And everyone knows not to spoil things by telling the future, except authors sometimes are desperate to capture the reader's interest . . . especially in the first sentence.
By the time the boy in Ward 4 attacked me, I'd already nicknamed him The Lost One in my head. He'd been admitted a week ago... (Leave No Trace, Mejia)
Later on in this book, when she's first working with him and things are going well, he suddenly attacks her. That sudden attack could have had a nice emotional impact for the reader. Actually, it would have, the author did emotional impact well. But once I was warned, the attack was expected and mostly fell flat.
Obviously, no author would mention the upcoming attack at the start of the scene, because that would ruin the surprise of the attack. And yet the author mentions it in the first sentence.
I'm going to be hit by a car in four hours... (There Will Be Lies, Lake)
There was no drama in her getting hit. There were teasers when I thought she would be hit, which gave me a feeling of dread (which did not match the main character's feeling), but then she wasn't. (The main character mocked me for having that feeling.) So I was relieved when she finally got hit and that event was out of the way.
I'm fucked. (The Martian)
After saying this, the main character backs up in time and starts telling how they were quickly evacuating from Mars, he fell and punctured his spacesuit, and almost surely he was going to die. But by not panicking and being clever, he survived.
Most authors would play that possible death for suspense. Maybe this author tried, but it didn't work because I already knew he survived. The only question was how he was going to survive.
By eliminating suspense, the author focused the scene on the main character's cleverness. Actually, the author does that all book. The same technique is used in 13 Reasons Why – we are told from the start that a character dies, so there is no suspense about that; the only question is why, and that's what the author cared about.
An important point here, part of my theory of spoiing, is that interestingness isn't spoiled just by knowing the outcome.
The Missing Scene That Would Have Been Spoiled
Back to Mars. After surviving, there was potential for emotional impact when, happy to be alive, he realizes he is left behind. Dismay! He's still likely to die in a few days, when one thing or another goes wrong. Bitter disappointment! But whatever potential that had was spoiled. In fact, the author didn't even try for an emotional impact. (I had to replay the scene in my mind to realize that potential was there.)
So, the opening line can spoil the emotional impact of a scene, and you might not even notice because the author already gave up on having any emotional impact.
The Law of Spoiling
Telling the reader what is going to happen is sometimes caller a teaser – something to interest the reader in what is going to happen. More darkly, it's also called a hook.
Ideally, you would provide a teaser/hook that does not spoil anything to follow. But I suspect this is impossible. Telling the future is a hook only because it has emotional impact. And the more it has, the more it spoils.
So, telling the future spoils.