I can imagine the frustration, but that never actually happens 😅
The wishlist always has 8 items and the inventory has 9, so you always have at least 1 slot to spare.
Also, as stated in the "controls" section, you can press "Z" to cancel so you can skip putting the new item in the inventory entirely.
In any case, I understand that it can be a bit confusing. The game was a bit rushed to be ready for Christmas and some details were left out. I'll make things clearer in future versions for sure.
Thanks for the kind words and for your feedback!