I've just had two scares playing this one while collecting star pieces. The first was on the moon, after I had beat Lord Crump on the moon and saved, I realized I could no longer get back to the moon (since the fortress didn't have any power for that), and hence I would be unable to go get the underground star piece I missed out there. But then I found out you can go back to the moon after you beat the Shadow Queen. The second was just last night when I went specifically on a quest for the star pieces – I was in Hooktail castle and realized I couldn't ride the yellow block up to get the star piece anymore because I already destroyed the stone block. But it turns out you can just fall from above to get it.
So my question is, what kinds of special collectibles (badges, shine sprites, especially star pieces) cannot be reached after a certain point? I've invested about 60 hours, so it'd be tragic if I can't go back and reach something. I know that in the previous Paper Mario game there was precisely one afterward-unreachable star piece: it's underground right before the Lava Piranha boss (after the volcano erupts, you can't go back to get it, which in turn means you can't get all of Merlow's badges). The fact that boss tattles can be found in professor Frankly's trash can, it doesn't keep track of enemies' first strikes, like the original does in Mario's house, and it seems to cares less about running away since it forces you to do it three times all suggest that TTYD is more forgiving.
Best Answer
I had a quick look through these FAQs:
It looks to me like no such collectibles are in places that you can render unreachable.
(Just watch out for this issue, which might nab you while you're searching.)