Twitter isn't shy when it comes to experimentation. The social network tests some new features by rolling them out to regular users, albeit on a very limited basis, while other projects, such as @eventparrot, are made available to the entire community. The latest unreleased tool to make an appearance is a notification window that pops up whenever you receive a reply or mention, a la the screenshot above. Engadget editor Sarah Silbert noticed the pop-up in her own account, but she seems to be the only staffer with access to this particular tool. We reached out to Twitter for more information, but reps weren't able to share anything related to this particular implementation.
The company has gone on the record before to shed some light on its otherwise mysterious experiment program -- in the past, engineers have "tested various features with small groups," and that appears to be the case here as well. Have you come across any Twitter features that your friends haven't seen? Let us know in the comments, and feel free to send us a tip with a screenshot, too.