Althought I've selected the latest version of WordPress above, I have been encountering this error for a while. The first instance I had was when, on one particular page on one of my sites, most of the content of a page was completely removed, leaving only one paragraph, every time I saved it. The only way I could get it to hang on to all the content was to use the HTML editor. That's the only instance I've had of stuff disappearing. Far more common now is at the bottom of a post or page in the Visual Editor, a random part of the existing text appears. I delete it, update it and it's back again. So far, I haven't been able to find any common denominator that is always present when this happens. It doesn't happen in every page or post on every site, but it is happening across all the sites that I work on. They all use customised themes that I have created either from scratch or using the HTML5-Reset theme, although that latter one is only on two sites.
Sometimes, the random text is just a closing </p> tag, sometimes it's a block of text, but as far as I can see, it is usually a copy of something that is already within the content.
Has anyone else experienced this at all?
Thanks for any input...