Websites that hijack well-known keyboard shortcuts to do things they aren't supposed to.
Notable examples: On a website, pressing delete takes you back a page. That's what the key does and it's often more convenient than clicking the "back" button. However, in Yahoo Mail, pressing delete deletes the message you're currently reading— and just to add insult to injury, it doesn't even return you to the message list as a result, choosing instead to open the next message automatically. Meanwhile, in any page of Google search results, pressing delete will automatically select the search query box and delete the last letter of your search query, automatically updating the results accordingly.
This wasn't always the case, but apparently Google's corporate charter requires them to replace every user interface every few months with one that's just a little more shit.
Meanwhile, anyone who has ever had to use a Mac knows that command-C is "copy" when using FruitOS. Except Microsoft (which is fitting enough), because in Hotmail, pressing command-C opens some obscure menu that no one ever needs and the only way to copy text is to select "copy" from the edit menu which no one ever does because it's stupid.
I'd give Microsoft a pass except they're a massive developer of Mac software so they should know better.
Apparently, the Hotmail command-C problem has now been rectified but only because Microsoft decided to rip off Google (in ways other than Bing) and has replaced Hotmail outright with "Outlook" which is exactly the same except that the user interface is pants.