We were seeing this as well, when we added a field merge to an email or by uploading HTML code that already had the field merges present. And it wasn't a data-entry issue either. And now it seems to have gone away on it's own.
Based on the timing, I suspect that it was an error that was introduced when they added WYSIWYG editing functionality to the Uploaded HTML editing screen. And then they fixed it without telling us about either the error of the fix. (Another error where extra line breaks kept getting added to the code seems, I hope, to have disappeared as well.)