We saw it happen with Harry Potter, with dire consequences - the first part of The Deathly Hallows effectively becoming Harry On Camping.
And we saw how, in doing this, Mockingjay Part One was made slow and leaden. And quite, quite dull.
But at least part two would be full of the bombastic action we so enjoyed in the first two films, right?
Bless 'em, they tried.
Unfortunately, there isn't enough action to spread out over two hours. So instead we get lots of attempts at drama and plot.
Attempts that feel like padding.
Across the two films, you get the action you're after, but in between you have to have all the star-crossed lovers guff which is better placed running alongside the action rather than being made the central theme.
At times you actually find your mind wandering as you wait for the next bit of fighting.
And that's not good.
Then there's slight problem of Part 2 picking up exactly where Part One left off with Peter back in the hands of the rebels and wanting to kill Katniss.
And if you can't remember Part One (and why would you?), you're playing catch-up from the get-go because it's just been blindly assumed that you know what's going on.
The plan here is a simple one. Storm the Capital, take out President Snow, save the day.
Snow, meanwhile, has turned the streets of the Capital into another Hunger Games arena, so there's lots of things to blow you up, shoot you or attack and try and eat you.
And those are the bits you're waiting for.
Sadly, the filming of the main battles are so poorly shot that you can't make head nor tail of what's going on until someone's name is yelled in pain-stricken grief.
But it's different to the first films, oh yes.
Because they're running about in the streets and sewers, not in the trees and lakes. So that's OK.
Only it leaves the main bulk of the film feeling like a computer game.
But let's be fair.
Maybe the death of Phillip Seymour Hoffman caused them a bigger problem this time round. Maybe it's in replacing the bits he should have been in that we hit the problem.
Or maybe, just maybe, in trying to stretch an average-sized book across four hours of screen time there just isn't enough quality.
And you can't blame the cast.
Jennifer Lawrence (Katniss), Josh Hutcherson (Peeta), Liam Hemsworth (Gale) and Donald Sutherland (who steals every scene as President Snow) all put in fine performances, even if Liam and Josh look slightly puzzled and/or bored at times.
This was a much-loved franchise, and I'm sure there will be fans who think this a fitting end to the sage.
But it's not.
With so few highlights, a semi-sombre trudge to the end, wrapped up with more final scenes than Lord Of The Rings, was no way to finish what had been a great saga.