Known-errors: tab-containers and rich-text-editor (documentation purposes)
When using rich-text-editor on a form with a tab-container (either within or outside the tab-container) at runtime entering data into the rich-text is sometimes possible, sometimes it is not possible. Seems entirely random. Solution: close the form and open it again a couple of times.
This problem and many others solved by replacing the Rich Text Auditor by the CKEditor.