Drupal allows only one editor to work on a page at a time. If multiple editors are working on the same page and subsequently save, Drupal will prevent the second editor from saving.
Working with Images
When copying content from an existing page, remember to re-upload images to the new site rather than using the existing image. Because the old site is still active, the image will appear to work fine, but will break when the old site is disabled after the new one goes live.
See the Recommended Image Sizes page.