When Dumbledore looks in the mirror Erised he sees his deepest desire:
>! Grindlewald

But before that the mirror shows the past when

>! Dumbledore and Grindlewald close their "blood troth"

In the Harry Potter books and movies the mirror is never shown to show the past. Is this a plot device as they had to show that important information somehow or is it a movie fail or simply an error in continuity?

Or does the mirror show the past of the person looking into it at any other time in previous books that I simply don‘t remember?

I know, that Harry sees his parents, but again this is his deepest need at that time and not a memory / glance of the past.

As for the duplicate identification: I specifically do not ask why Dumbledore sees 

>! Grindelwald

as this is very well answered in the other question. But I want to know why the mirror shows that specific point in the past to "explain" the desire to us spectators. It did NOT explain Harrys desire for his parent using old memories, it just showed him standing between them (although of course they looked like they did in the past as there are images that are more current than these)

One could say: The desire of Dumbledore is, to revert the troth. But then imho he would see 
>! the vessel being destroyed

in the mirror and not it beeig created...

That's the reason for my question: Is it a movie error to show the mirror showing past events that explain the current desire of the person standing in front of it? Or is the mirror shown to behave like that in any other canon source?