While working with invisible reference image and segmentation nodes I encountered the following:

1. Open MITK and load an image.
2. Open the Segmentation view.
3. Select the loaded image in the Patient Image selector.
4. Create a new segmentation node (Tools will be enabled).
5. Hide the segmentation node. A warning appears (Tools will be disabled).
6. Hide the reference image node
7. Show the segmentation node. The warning disappears (Tools are still disabled)
8. Show the reference image node (Tools are still disabled)
9. Reinit either the reference image node or the segmentation node. The tools will be enabled.
10. Hide the reference image node (Tools are still enabled)

I see step 7 and 8 as problematic: No clear instructions on what to do are given. Outcome of step 10 might be required, but at a first look it looks questionable (drawing without visible reference image).

Another similar problem:
Perform step 1 - 4.

5. Add another image and create a segmentation node for it.
6. Hide the first reference image and segmentation node.
7. Change the patient image and segmentation selection to the first pair of nodes (the hidden ones). A warning appears (Tools will be disabled)
8. Show the segmentation node again. Another warning appears (Tools are still disabled).
9. Reinit the segmentation node, as the warning suggest (Tools are still disabled).
10. Show the reference image node (Tools are still disabled).

Reinit in step 9 does basically not have any effect if the reference image node and the segmentation node are not both visible at the same time.