If your timebox only allows for one iteration, then you're going to need three timeboxes: one to do it, one to learn how you fucked up, and one to redo it.
If you’re doing your work properly, you’ll do it at least 2.5 times.
The first time you build a document, project, character, sound, whatever it may be, it won’t be great. You’ll probably restart from scratch.
The second version, it’s better, but you have to re-iterate half of it.
This is normal.