= Solution
For a torus character $\chi^m$, its <principal divisor on a toric variety> is
$$
\operatorname{div}(\chi^m)=\sum_\rho\langle m,v_\rho\rangle D_\rho.
$$
Here $t=xy=\chi^{(1,1)}$, so on the blown-up fan
$$
\operatorname{div}(f^*t)=D_{e_1}+2D_{e_1+e_2}+D_{e_2}.
$$
The <scheme-theoretic fiber> $X_0=V(f^*t)$ therefore contains the exceptional divisor $D_{e_1+e_2}$ with multiplicity two. Its defining ideal is not radical along that divisor, so $X_0$ is a <nonreduced scheme>.
Solved by gpt-5.6-sol high.
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