macOS
7 curated resources, easiest first.
After a macOS upgrade, git and other developer tools often stop with this error because the Command Line Tools need reinstalling. Running xcode-select --install brings up an installer, and you don't need the full Xcode app.
macOS reorders Spaces based on which you used most recently, which confuses keyboard switching. Turn off 'Automatically rearrange Spaces based on most recent use'. It's in System Settings → Desktop & Dock → Mission Control on Ventura and later, or System Preferences → Mission Control on older versions.
Cmd-` (the key above Tab on US keyboards, left of Z on UK ones) cycles through the open windows of the front app, and Shift-Cmd-` goes backwards. The answer explains it only works for windows in the same Space that aren't full screen, and how to get round that with Cmd-Tab.
kernel_task deliberately takes CPU time to slow the Mac down when it's too hot. This answer traced it on Intel MacBook Pros to heat from charging on the left-side Thunderbolt ports, and found charging from the right side helped. Mostly relevant to Intel models.
With multiple displays, the Dock moves to whichever screen you push the pointer down against the bottom edge of, so it often moves by accident. To bring it back, move the pointer to the bottom of the screen you want and keep pushing down.
macOS's netstat doesn't take Linux's -p flag for process names. The answer gives a netstat filter for listening TCP sockets, and an lsof command that shows the program name and process ID for each listening port.
AppleScript's built-in display notification command shows a native notification without any extra apps. From a shell script you can call it through osascript, which is handy when a long job finishes.