TITLE: Bounded extension
QUESTION [6 upvotes]: What are the easiest examples of a pairs of Banach spaces $X,Y$ such that 

$X\subseteq Y$ ($X$ is a closed linear subspace of $Y$)
there is a bounded linear map $T\colon X\to Y$;
there is no bounded extension $\hat{T}\colon Y\to Y$ of $T$?

Needless to say, I am interested in the structure of the operator $T$ rather than in its existence.

REPLY [3 votes]: Consider $X = C^1(0,1)$ and $Y = C^0(0,1)$ and the map $T = \frac{d}{dx}: C^1(0,1) \to C^0(0,1)$. Every densely defined unbounded operator will provide an example.