Ran across this post after suffering with the same issues. I just accidentally deleted half of a days work in Evernote because I pasted some html into a note when I had most of the text within the note highlighted. Just moving too quickly and didn't notice I had the previous text selected. This action of course replaced the highlighted text with the html content. Normally no big deal... hit ctrl-z a few times and your off and running. However, cntl-z does NOT work after pasting any html content. I think it may be setting the Source URL in the note and after it does this, the undo stack is inaccessible.