As I've learnt recently in my Functional Analysis course, it is well known that if $X$ is a normed Banach space and $Y$ is a closed subspace, then the quotient $X/Y$ is a Banach space (e.g. How to show that quotient space $X/Y$ is complete when $X$ is Banach space, and $Y$ is a closed subspace of $X$?)
However, I've been trying to find an explicit example of a normed Banach space $X$ and a non-closed subspace $Y$ such that $X/Y$ is not a Banach space, but I haven't come to something yet.
Can you help me to find such spaces? It would be great to read your answers, there may be some interesting examples out there.