you get a very disorienting "why won't it slow down" feeling, and it's easy to panic.
You don't always even panic. It's weird: if you do something in muscle memory enough, you don't consciously think about doing it. This is why driving etc is smooth because you aren't thinking about every action. When something breaks [see what i did there!] you at best get a creeping feeling of wrongness that takes a while to percolate up to your conscious brain.
I can relate a few anecdotes.
One typically dismounts a bike by first lifting your foot up off the pedal before putting your foot down. After an accident where I slipped off a pedal climbing a hill, I switched to toe clips, which require you to tilt your foot down then slide it up and out. Lifting doesn't work. First time I came to a stop, I toppled over into a puddle while hauling my foot upwards, but not even thinking about it. I wasn't panicking because I hadn't even noticed until I was at about 45 degees at which point I had a brief flash of "oh shi..." before landing in the puddle. But it's funny: lower level part of my brain wanted to lift my foot and just kept applying increasing force to match the requirement, but didn't inform me.
Next (harmless) was on moving to the US. I was driving along and then my partner asked me what was wrong, which confused me. She then asked why I was batting at the door. I had *NO* idea I was doing that. I'd only ever driven a manual on the other side of the road. Based presumably on engine noise etc, my hand was searching continuously for the nonexistent gear stick on the wrong side of the car. The weird thing was it was going all of its own accord. And because nothing had gone really wrong, nothing jogged my low level brain out of that loop, so it was happily searching forever, and my conscious mind was completely unaware.
Third was in winter, wearing heavy boots driving an unfamiliar rental canyonero, and clipped the gas with the side of my boot while braking. The car wasn't slowing properly, and the low level feedback control part of my brain just kept commanding more force, so I kept kitting both pedals ever harder. I got a creeping feeling of wrongness as the car wasn't stopping, but it's basically of the form "why is my limb not working properly".
I then rear ended someone, and I can't remember how I realised what the fuck I was doing. Anyway the guy was really nice.