Exercise C6 We have

0 ⟶ ℤ/2ℤ ⟶^{i} ℤ/2ℤ ⊕ ℤ/2ℤ ⟶^{p} ℤ/2ℤ ⟶ 0
≅↓  ≅↓  ≅↓  ≅↓
0 ⟶ ℤ/2ℤ ⟶^{×2}  ℤ/4ℤ  ⟶^{π} ℤ/2ℤ ⟶ 0

with exact rows, but ℤ/4ℤ and ℤ/2ℤ ⊕ ℤ/2ℤ are not isomorphic.