Okay, heres the bug.

I created about 30 frames for a sprite, and they were positioned in the top left of the costume editor. They were all in the correct position online.

In the offline editor, the frames run through very smoothly in the stage. If you go in to the sprite and click on costumes, one of the costumes changes it's size from the correct size, (something like 123x45) to nothing. (1x1) The actual costume, whichever it may be, (it is random) moves it's position from the top left, to the bottom right. If you re-press the green flag, the frames run through fine until it reaches the affected costume(s) and the sprite appears to jump briefly to the bottom right of the screen, and then back to the top left again, in just a split second.

Please help because it can be very annoying when you dont have an internet connection and can't use the either editors.

Thanks for reading,
Jack.