Gmail - When you send an email that contains a Google Drive link, it warns you if the file isn’t shared with the recipients.
Amazon - Wish List will notify you that someone may have purchased an item on your wish list if you click-through to the item’s page.
Facebook - The notifications icon shows a different side of the globe depending on your location.
/via Thomas Park
Twitter - When submitting the same tweet twice, an error message is shown.
/via Sindre Sorhus
Foursquare - In the iOS notification settings, the option to include adorable puppy photos in weekly updates is given.
Quora - If someone is answering a question you are currently looking at, a little notification pops up telling you so.
/via Allison Ko
Flickr - Error message when you exceed the 140 characters limit when posting a photo on Twitter.
Kayak - Gives a warning if you try to book a hotel for a check-in date that coincides with your existing reservation.