I was pasting some code for a page and discovered that the Visual Editor was adding code not placed there in the HTML Editor. I pasted first in the HTML editor and then previewed in the Visual Editor. I noticed a discrepancy and then went back to the HTML Editor only to find added code.
The only work-around I found was to paste the code in the HTML editor and NEVER preview in the Visual Editor. I hit Update as soon as the code was pasted into the HTML Editor and then navigated away from the page. All is well as long as I never preview in the Visual Editor.
For some reason the Visual Editor kept adding <tbody> tags.