To reproduce on the API docs website:
1) Make a board+card+checklist+item(s)
2) Go to https://developers.trello.com/v1.0/reference#checklistsidcards and enter your key and token and the checklistId. (This works.)
3) Go to https://developers.trello.com/v1.0/reference#checklistsidcardscheckitems and enter your key and token and the checklistId and one of the checklistItemIds from step 2. (This does NOT work.)
4) Metadata contains:
REQUEST DATA
"invalid value for idCheckItem"
STATUS
400 Bad Request
It's not a problem with the docs page, as I get the same result when calling the same endpoint from my server and browser.
ps. I tried clicking the Suggest Edits link in the API docs website, but it went to a 404 page :-(