* Default light and dark themes of programs/apps and sites that have illegible low-contrast color combinations. Very common, sometimes within themes they control entirely but apparently never tested both light and dark themes. (For example, the non-customizable foreground color a calendar program users to indicate task priority doesn't change when the theme changes the background color to dark. Or the terminal app thinks that yellow is a foreground color to use for highlighting warnings, and apparently the author has never heard of terminal windows with white backgrounds.)
* Bluesky's delayed page-loading full-screen interstitial of small butterfly logo against white backdrop, rendered when they know the session is in dark mode. If you've in dark mode in a dark room, winding down for bed at night, this is awful. I occasionally notice lesser problems like this with streaming video UIs, but not as consistently jarring/blinding as Bluesky. (Almost as awful as the movies/shows on home video streaming services that throw in gratuitous strobe light scenes. If it doesn't give the customers seizures, it stil can't be good for their sleep cycle. At least Bluesky's unnecessary sudden blinding white doesn't strobe.)