Plain Text Editing
Beneath the editor is a button labeled Text Editor. Click this button to edit the page using a plain text area (no rich text formatting). This is useful for managing content that the editor will not accept, such as highly complex HTML, Javascript, and IFRAMEs. You can then include the plain text page into a normally editing page using an include tag.
The Code Editor button behaves similarly, but attempts to improve upon the plain text editor by adding syntax highlighting and line numbers. The Code Editor can be a little fussy - switch to the Text Editor if you're having troubles with it.