For more than a decade now, I have been using the Tab key to demote a title in Outline view, and Shift + Tab to promote it. This almost always works.

However, in some rare instances, it stops working, and indents the text instead, without changing the outline level. Things usually come back to normal a while later.

I've never been able to find what changes this state. Actually, I have never found documented the demoting/promoting action of Tab and Shift + Tab. All my Word books mention instead Alt + Shift + right and left arrows, which I have never used.

Can anyone help me solve that mystery ? Thank you.