For a single-diode half-wave rectifier going into a capacitor filter, the peak repetitive inverse/reverse voltage on the diode is TWICE the peak input ac voltage. If you exceed the PIV/PRV and short the diode, you could damage other components (transformer, etc.) and maybe start a fire. If the diode opens up, your output dc voltage goes to zero. This could even happen if you don't exceed PIV/PRV.