The Stacks project

Example 109.7.1 (Non-reduced automorphism group). Let $k$ be an algebraically closed field of characteristic $2$. Set $Y = Z = \mathbf{P}^1_ k$. Choose three pairwise distinct $k$-valued points $a, b, c$ in $\mathbf{A}^1_ k$. Thinking of $\mathbf{A}^1_ k \subset \mathbf{P}^1_ k = Y = Z$ as an open subschemes, we get a closed immersion

Let $X$ be the pushout in the diagram

Let $U \subset X$ be the affine open part which is the image of $\mathbf{A}^1_ k \amalg \mathbf{A}^1_ k$. Then we have an equalizer diagram

Over the dual numbers $A = k[\epsilon ]$ we have a nontrivial automorphism of this equalizer diagram sending $t$ to $t + \epsilon$. We leave it to the reader to see that this automorphism extends to an automorphism of $X$ over $A$. On the other hand, the reader easily shows that the automorphism group of $X$ over $k$ is finite. Thus $\mathit{Aut}(X)$ must be non-reduced.