Reproduction steps:
1. Open attached project
2. Open scene "test"
3. Select the gameObject "Text" (under Canvas/Panel/Text)
4. Switch into rectangle mode by pressing "t"
5. Drag the upper border of "Text" so that the upper border would be under the bottom border
6. Select "Panel" (under Canvas/Panel)
7. Drag the upper border of "Plane" below the bottom border.

Note: The error only occurs when the text has a "Shadow" component attached and when its vertically maximized (anochorMin.y = 0 and anchorMax.y = 1). Some times to reproduce the bug you need to drag the upper panel back in place (above the bottom panel).