bug#16377: Undo Tree regression: (error "Unrecognized entry in undo list undo-tree-canary")

[Stefan Monnier]

> Are there circumstances in which `primitive-undo' doesn't add to
> `buffer-undo-list'?

Of course: if there are no changes until the next undo boundary, then
primitive-undo just doesn't do anything other than pop that
undo boundary.
And primitive-undo doesn't add any boundary, so it can end up with no
change at all to buffer-undo-list.


        Stefan