Very consistently. Happens 100% of the time.
Embed a GDocs spreadsheet into your post, uising the following code as an example:
<iframe width='600' height='350' frameborder='0' src='https://docs.google.com/spreadsheet/pub?hl=en_GB&hl=en_GB&key=0AqJjQktZy8GkdHZfdkdxeUl0WjVVUDVtN0s3REM2TlE&single=true&gid=8&output=html&widget=true'></iframe>
Now submit your post (or, go from the source HTML editor back to the RTE, then back again to the HTML; both do the same thing), and you'll notice the code has changed to:
<iframe frameborder="0" height="350" src=https://docs.google.com/spreadsheet/pub?hl=en_GB&hl=en_GB&key=0AqJjQktZy8GkdHZfdkdxeUl0WjVVUDVtN0s3REM2TlE&single=true&gid=8&output=html&widget=true width="600"></iframe>
Two things are now broken:
1) The width="xxx" has been moved to the end of the <iframe>, and height="xxx" has been moved to after frameborder="x". This breaks the embed every time. This means you can embed a spreadsheet, but editing the source afterwards breaks it, and you need to manually fix things.
2) All the "&" have been converted to "&". Doesn't actually break the embed, but not expected behaviour.
I didn't expect the variables in the <iframe> to move about.
Windows 7 x64 Professional
Any post which includes a spreadsheet embeded using the RTE.