It's not the worst thing in the world, but it isn't fun.
It happened to me once. My PC beating back the bad guys pretty effectively, so the DM altered the rules mid game so that a spell could reach my PC and knock him overboard. ..and of course there was no way for me to climb back up.
This echoes a sentiment stated somewhere up thread, and speaks to some rather disheartening experiences I've had around agency that, at a glance, don't seem related to the base topic of the thread but share some themes.
My Friday night game is a bit of a potpourri where everyone takes turns DMing and runs their pet thing. Lately, it had been a lot of D&D 5e (likely as it's a lingua franca and the one thing we could all agree on). The stars had aligned recently with two of the players with seemingly similar predilections running 5e modules: one wanted to run Vecna, and the other Frostmaiden.
One had exhibited some...capriciousness as a DM in the past, and the other hadn't, though both tend to be a bit "aggro" as players: doing stuff that veers the rest of the group off course. In both these instances, the DM v. player paradigm came out to the fore.
In Vecna, the DM lectured us for not being specific in our searches. I get that there's a backlog of old skool searching vs. player skill but the rub is this: when I searched a shield in a tomb (thinking it might have a clue), the DM immediately changed the curse rules to inflict it on me for merely interacting with it. So, on the one hand, he wants us to search granularly. On the other hand, it was clear that it was a way to make it seem like our choices were what inflicted the penalty. It took away agency and made the experience kind of...arbitrary...which is antithetical to why I play RPGs. Had the shield been merely trapped? I'd have been okay with it. If it had killed my character or put me out for a minute, that would have been okay, too. It was that stripping of agency mixed with the implication that it was my choice (agency) that had done it that bothered me.
Something similar happened the next week in the Frostmaiden game. That player who ran Vecna played a character that basically antagonized everything and everyone we came across, making it difficult to make meaningful choices or interact with the setting. Worse still, the DM clearly wanted the early bounty on the possessed murderer to become a scenario in which we were framed for the murder we were trying to prevent.
We staked out the merchant caravan (I later learned that in the module the PCs know who the suspect is straight away; in this version, we spent a lot of time observing and narrowing suspects). A group of us waited behind the rogue who would be best able to follow the suspect. We would follow the rogue from a slight distance. That's when things went south, quickly.
Though we had discussed the plan, the DM asked what we wanted to do after the rogue began following the suspect. He had the first character who responded that we were following to make a single Survival roll to see if we could keep up. A "14" failed. This is when my spidey-sense went off. Tracking someone in the wilderness is normally a 15 DC. We watched the person we were following (and who was helping us follow him) disappear. In a snow laden village. Several entrapping sort of questions followed--"Do you want to knock loud enough for others to hear?" culminating in a confrontation where the suspect revealed his ice blades and fought us. Apparently enough town folks saw us attacking him to make us suspects, but somehow no one saw his weird magic blades. Like the previous village, we had to flee into the country. Again: we were stripped of agency mixed with the implication that it was our choices (agency) that had done us in.
Several of us had become pretty sullen by this point, and both DMs have kind of cottoned to the fact that people weren't having fun. But neither of them copped to the idea that maybe--just maybe--the way they structured the experience was the reason rather than mere player petulance. I want to point out that this is in a group of 30-50 year olds (and working professionals).
I've had several characters die, go insane, and been maimed in games, and
had a blast because it fit within the milieu and were either the result of my choices or just bad luck. I don't mind losing an occasional turn due to a spell, but I should hope it's part of an encounter that ends soon enough for me to participate in the game again and not a result of the DM trying to "win," whatever that might look like. What I've outlined above, and what I think other posters have gotten at, is when I'm not really participating but instead being implicitly told that my choices are why I'm not participating.
We're now playing video games together instead and maybe having another player run not-D&D, is that seems to be the excuse everyone is using for why it didn't work out. Both of the culprits are fun outside of those contexts, but sensitive and not terribly receptive in the past to constructive criticism.
That's all to say that while the question is ostensibly about mechanics--defining things in terms of a round--I think more deeply its about agency and the fact that at the end of the day an RPG is a social act. How we define that agency and come to some mutual agreement about what social rights participants have in relation to it is pretty crucial. And apparently some extraordinarily intelligent folks in my social sphere (myself included) are having some difficulty navigating that.
