In the structure of diborane ($B_2H_6$), each boron atom is $sp^3$ hybridised. The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four terminal B-H bonds are regular two-centre-two-electron (2c-2e) bonds, while the two bridging B-H-B bonds are three-centre-two-electron (3c-2e) bonds (also called banana bonds). Therefore, the statement that both boron atoms are $sp^2$ hybridised is incorrect.