Just now, I had taken some screenshots in a game. One of the screenshots insisted on sitting outside the Screenshots stack for some reason (Stacks automatically collapse similar items on the desktop.)
When you open a book in Books.app, then view a different Desktop Space, the Books window automatically jumps to that space, even without the "Show on All Desktops" option.
Activity Monitor column widths randomly grow towards infinity.
Display Accessibility options randomly stop taking effect until relogging.
Widgets randomly rearrange themselves in the Notification Center.
Music/TV/Podcasts/App Store sometimes show no content, just blank views.
On the developer side, many bugs and missing documentation for SwiftUI.
I don't have the energy to type them all out here again after writing their bug reports.
If that doesn't happen to anyone else, I wonder if it's my iCloud-stored settings for Books.app