Why does my site break when I paste in content from Microsoft Word?
Microsoft work adds proprietary information to provide visual formatting to documents. When you copy and paste content from Word, this formatting information comes along with it and causes the problems you're seeing.
To fix this, use the "Paste from Word" button in the editor (looks like a clipboard with a W in front of it). This will remove the problematic style information.
Another method that will prevent this problem is to first paste your content from Word into a text editor such as Notepad. Then re-select your content in Notepad and copy and paste it into the editor. This extra copy-paste step will remove the extra code that comes from Word.
Another option is to use the "Cleanup messy code" button (looks like a broom). Sometimes this will solve problems with extra tags that get pulled in from word processors.