When finishing dream I-18, you are given a blue key (apparently useless) and you are told to discard all cards from the dream (as there is no "old dream" gate for that).
When going back to dream I-15, upon using the blue key you are told to reveal card D, but this card from this dream is not related to the occuring event. It seems that you are supposed to reveal card I-18 D but it has been removed from the game.
As that card is very important to continue, I am going to assume it is an error, and take that card back from the removed ones.
Am I right to assume that this is an error?
BTW, I am not a huge fan of that extra step of getting that blue key instead of directly getting the I-18 D at the end of the dream, especially as there is no clue (as far as I am aware, I might have missed it) on what to do with it. It seems like an extra step in which you have to randomly roam through the old dreams in order to find the right action.
Other "hidden stuffs" that could block you are clearly stated in the end of the "train station" dream and the "eternal city" one. It feels better that way as, though you don't know what you are looking for or where (and some hints are available in other dreams, which I found great), you know you have to find something there!
Rest of the game is amazing, though! A lot of variety in the dreams.