Hi Martin -
I've had this problem a couple of times. I think it might have to do with the number of characters pasted into the editor, but I'm not sure. It looks like some of the characters disappear from the end of the text, messing up the entire thing and horribly breaking the HTML - sometimes with disastorous results for the page.
I have attached a file that one of my users had tried to add into the editor using cut and paste. This is an entire HTML document created with Frontpage (gasp), but if you just cut and paste the main table, you may see the problem (I did). It looks fine when you first paste it, but when you save it, it gets really messed up.
The document itself is poorly formatted (uses specific widths) and contains some Arabic text as well. When I used Word to clean the HTML, it would paste properly, but the formatting was even worse. This process also reduced the number of characters.
Anyway, see what happens when you try it.
mike