What happens when face-hardened armor is reversed

How serious would it be if face-hardened armor were installed with the hard side inward?

Very serious: the plate would resist penetration less effectively and the inward hard layer could produce dangerous internal spall.

Reversing face-hardened armor changes how the plate absorbs a shell's impact. The projectile would lose less energy in the intended manner and would be more likely to penetrate and explode inside. The inward-facing hardened layer could also break into fragments and send dangerous spall into the protected space, even when the shell did not pass completely through the plate. The harder face and more flexible backing therefore have different jobs, and their orientation matters to the crew behind them.