When I selected a sprite with the intention to move it, it was picked up, but translated hundreds of units to the right. So, when I moved my mouse, it moved, but it was always on the right-side, off of the screen. Then, when I dropped it, it was still off-screen. It can still be recovered by simply telling it to move to the confines of the screen. This happened on the online editor. Something interesting is that I can still interact with it whilst it is off-screen; it still does everything that I programmed it to. I believe that it is cause when a sprite grows in size whilst you're holding it / start holding it. When I pick it up during its growth, it sometimes rockets off to the right.

PICTURES:

https://imgur.com/qQM6ujG
https://imgur.com/wnXLyvA

My browser / operating system: Windows 8.1, Firefox 58.0, Flash 28.0 (release 0)