The little [tick/seen] notification that lets you know people have received your plea for help/intervention is making its way to new parts of Facebook, with the social network currently adding the looked-at receipt system to event invite notifications for a small testbed of users to try.
In short, when someone you don't really like who's always overstepping boundaries and inviting you to things you never attend, the sender will now know you actually read the invite. So saying you didn't click on it or see it or that it somehow scrolled away and out of view under a sea of similar requests seeing as you're so popular, won't work any more.
Of course, unless Facebook can access your webcam data, it also won't know that your eyes actually focussed on text box containing the words, so there's always that excuse.
Plus there's the chance this feature won't even make it out to everyone, as it's currently only a test scheme at the moment. Facebook told the Daily Dot that it's only a trial, explaining: "We’re piloting the ability for private event hosts and guests to know whether their friends they invited have seen the event." [Daily Dot, Twitter, via Mashable]