Things I've noticed are wrong with my Mac.

The "installed" noise it makes when you install something from a .dmg sounds like an "uh uh this didn't work" noise. I thought this was just one particular installer, but nope, it seems pretty common.
The Macbook's USB-C ports gradually fail to charge. One after another they break, and you have to use a different one. They all still work for data, but charging... nope.
Today's notification is that an upgrade to "Big Sur" will happen overnight tonight. I do not get a choice in the matter this time (though I have before). When I click for more information it says it's going to download and install "Monterey". There's nothing to say which is actually going to happen, or which is the newer version since it doesn't use version numbers in this dialog.
After opening RDC and Photoshop in quick succession, the global menu has turned into a mess of both. Top level we have File-Edit-View-Help, but the dropdowns are from photoshop. The Apple menu is empty. If I click the empty space in the menu bar I get dropdowns for RDC or Photoshop seemingly at random.
Spotlight doesn't find all installed apps. For example Xcode is fine but not iOS Simulator - even though they're both in the dock.
Time machine lists all mounted drives and lets you select any of them, but the next dialog doesn't have a continue button if it's an "incompatible" drive, such as a FAT32 NAS box. No explanation is given.
Similar-role dialogs sometimes have "CANCEL" buttons and have their window controls disabled, and sometimes have active window controls but no CANCEL button.
Some of the keyboard shortcuts reference symbols which don't appear on the keyboard ("Force Quit", for example) The shortcut reference symbol for "ctrl" is ^. While common knowledge among command-line users, it's not shown on the keyboard and is confusable with shift-6.
What is "make alias"? If you right click a mounted drive in Finder, it's an option. All it does is say "Error -8060".
Sometimes you can't move a window from one workspace to another by pushing it against the screen edge.
Preview sometimes gives you the option of opening the file with its default associated application. Sometimes it doesn't.
Preview reloads its file when you move it from one monitor to another. No other application does this.
AppleMenu » Restart often doesn't work, with no error. Sudo reboot from the command line solves this.
Update notices use a completely different popup notification to any other app.
Update notices can't be dismissed, if you click on them it opens the details page.
Updates often say "you can't update to this blah blah because your version is blah blah" when you click on them. You have to install them in the correct order, which is not indicated in any way.
"Incompatible" updates refer to their required version only as a number (e.g. 10.9) whereas OS updates refer to themselves only by name (e.g. "Mavericks")
Home and End keys sometimes take you to the start/end of the line, sometimes to the start/end of the document, sometimes do nothing. No clue from context.
You can't "cut" a file in finder. It's an option in the "Edit" menu, but it's always greyed out. It's not an option in the context menu at all. Consistency?
"Your computer must be plugged into AC power for the update to install" on some software updates. Apple knows whether this is a laptop or desktop, an IF condition here would have been better than a dialog box full of nonsense.
With a "maximized" iterm and an auto-hidden dock, the terminal shrinks by the size of the dock whenever the mac comes out of standby.
iTunes.
When waking up from sleep, the password dialog doesn't recognise the first character if it's shifted. So if your password starts with a capital letter you have to wait for 5 seconds or so before starting to type or you have to get used to it giving you the access-denied shake
When waking up from sleep, sometimes Terminal starts handling mouse events (clicks, scrolling) by filling up with escape codes. Sometimes it works.
The app store hides the download button for "free" items behind a button labeled "free". Nobody knows this unless they've had to google for it before.
"Free" apps on the app store require an Apple ID to access. The Apple ID says it's free, but won't let you complete the download unless you provide valid credit card details.
Sometimes all apps will stop accepting text input, and the text cursor will disappear for a minute or so.
Fanboys.
After upgrading to Mavericks the multiple-monitor support has changed so you can no longer drag a window from one workspace to another.
The default app associated with .txt files is TextEdit. Guess which file format it won't save in through File -> Save As? You have to open the .txt file, choose Format -> Make Plain Text and then File -> Save.
Upgrading to Mavericks removed the xcode cli tools, without telling me, which means brew no longer works. xcode-select --install installs them again. As soon as they're finished downloading, brew gives me the message "A newer Command Line Tools release is available. Update them from Software Update in the App Store." - but there's no such entry.
The upgrade to iMovie, an app which I didn't choose to install in the first place, is 1.97 gigabytes. Giga. Bytes.
If you use keyboard shortcuts to slide between multiple workspaces, go to space 3, then cmd-tab to an app on space 1, where would slide-right take you? Did you expect space 2? No, it's space 3. Behaviour is different depending where you've cmd-tabbed from most recently. This is confusing on a scale I didn't previously believe possible.
Finder lets you open a terminal from a folder, but only by right-clicking a folder, not from within it. This option is not available by default. Enabling the context menu involves navigating through System Preferences -> Keyboard -> Shortcuts -> Services and checking a box there. This dialog and the feature it enables are so disconnected it might as well be in a locked filing cabinet. "Beware of the leopard" has never seemed so apt.
Finder doesn't show you all files and folders. You can't see /tmp, for instance, even when selecting "All Files" and viewing the top level of your drive. You have to use hacks like ln -s /tmp ~/tmp to get around it.
Touch ID stopped working after an update. It no longer prompts me to use my fingerprint to unlock the machine, and when I look at the preferences, and hit the big plus to add a new fingerprint, it says Failed - Unable to complete Touch ID enrolement. Please go back and try again" without prompting me to use my fingerprint. It gives me the options "Cancel" and "Start again", both of which display the same message.
The keychain no longer recognises my password after an automatic update. The recommended solution, according to on-screen prompts, is to create a new, blank, passwordless chain and set a password by using "reset password" from the Edit menu. There is no "reset password" in the Edit menu for the Keychain app.
Some applications which crash a lot, like Docker Desktop for example, don't appear in the "Force Quit" list. You have to kill them from the command line - or if you're not a command-line person, you have to reboot like some sort of animal.
On the occasions where Force Quit appears to work, it often leaves behind the application's global menu icons, and if you hover over them you see the spinning beachball thing. There doesn't appear to be any way to fix this.
If the MacBook shuts down overnight, it's impossible to tell whether it failed because it ran out of power or some other reason, since it has no indicator lights and takes a minute or so to show any activity after pressing the power button. Today I changed the charging cable to see if that was the problem only to find the machine spontaneously power up and say it had over 80% battery charge.
The prompt to send an error report to Apple crashed, and I had to resort to force quitting it after a few minutes of beachballing.