Scratch

Memory. It only seems to happen:
-in big projects with lots of scripts
-in projects with lots of media
-when other applications are running such as IE
-on slow computers

All of which take up a lot of memory.
Here's my theory: I think that when memory is running low, scratch tries to optimize available memory. I think it clears some cache that has to do with text that can either be directly changed or could be changed by scratch when you do something. All the data is still stored, except for the display, so you see an empty box. So if it needs to be redrawn, like if you put something over it, it can't find anything to draw it with. That may explain why saving it, or closing and reopening it, restores the text as it has to load the data from a file and and build the GUI for the project.

I haven't actually read the scratch source code so I cant say for sure, but it is my theory of what happens.
Hope we can get it fixed!