Whenever I preview a Model in Toolbox, that Model gets inserted into Workspace. This can be very dangerous, as it may let in a virus into your game without you even knowing it, as you can’t preview the Model without inserting it (which is what the preview feature was all about when it came out).
To replicate this, open Toolbox, hover over a Model and click the 
icon, and you should notice the Model get inserted, even though you clicked only on the preview button.
(don’t mind the misplaced mouse, the different mouse icon must’ve messed it up a little on the recording)
Restarting Studio doesn’t fix it, but I can try reinstalling later. Let me know if this bug does/doesn’t occur for you.