This is a bug that's been around since at least 1.0:
Page -> Dialog -> Dialog <- Back <- Back -> Forward -> Dialog <- Back
At this point, it should go back to the original page, but it stays at the dialog, which can no longer be closed using its "Close" button, because there's code that prevents multiple clicks of the "Close" button from behaving differently from a single click of the "Close" button.
In the above sequence, when you go forward, it skips past the first stale dialog entry - which is correct, but there are nothing but stale dialog entries all the way to the end of the history.
We can generalize this like so:
Initial -> Unreachable 1 -> Unreachable 2 <- Back <- Back -> Forward -> Unreachable 1 <- Back
Once we're done re-working nav, we should make sure this sequence works.
Closing this dialogs are deprecated in 1.4