Scratch

Before posting an error please look if this error has occurred with other people. There is a sticky about this in the troubleshooting forum, here is a link: http://scratch.mit.edu/forums/viewtopic.php?id=21219

It would be good if you deleted this, because loads of other people have also reported this. But I'll tell you what to do. The bug usually occurs when you have lots of things running in the background or you have a project with a large file size. To get rid of it, save it, then go off, then open it up again. What you need to do then is go through every background and costume, click edit, then without doing anything, click 'ok'. It takes quite a long time, but it works. After this, save your work again, just in case it happens again.