All concrete collapsing, massive drops/increases in pressure that can shatter your eardrums, untreated metals cold welding, full UV rays going through an Ozone layer that no longer exists, planes fall out of the sky… oh and yeah the Earths mantle just fucking collapsing in on itself and essentially dissolving as its composition has an enormous amount of oxygen
I don’t think that’s that bad, especially because the pressure drops everywhere equally. Most planes would be able to work just fine, they’d just have to fly a bit lower, if even that. Divers would get squashed though.
Any container holding pressure that isn’t just oxygen would rupture because they were designed to work under atmospheric pressure not 80% atmosphere.
Things like hydrogen, natural gas containment and basically any kind of balloon would not have a good time.
A sudden drop in outside pressure would be devastating to many structures because dynamic load from the gas slamming against the vessel walls.
Conversely anything containing a large amount of oxygen would suddenly be a vacuum instead of a positive pressure environment and would certainly fail.
Not to mention all the gasses dissolved in water would want out because of the decreased pressure.
So things like your blood would probably release nitrogen and give you an embolism which is especially problematic because it won’t just instantly go back in once the O2 returns. When divers get the bends it’s from this effect and it takes hours in special pressurized chambers to come back from it.