Perhaps this word doesn't exist. I'm looking for the word to describe the last final thing you want to put in a container but it won't fit. There is always one of those. You have ten items but always only nine will fit into the container you have. Often occurs with a toolbox which has one too many tools to go into it.
In my family, although I'm not sure when it first originated, we've always used the term square peg. Such as:
Figures, get the suitcase basically done and last thing ended up being a square peg.
It just comes from the old thing of a square peg trying to fit into a round hole in the children's block game. Can't force it through the circle slot no matter how much you want it to fit in. I am aware this is a pretty silly way of using the old idiom loosely, but for some reason it always really just felt like the perfect thing to say ever since I first heard my Grandfather use it when I was a child.
Perhaps a little too formal, but supernumerary fits the bill:
Something which is beyond the prescribed or standard amount or number.