Chipping or flaking of the inside of the tank due to things hitting the outside. During World War 1, tank crews were issued chain mail face masks to help ameliorate the effect. The mask would look something like this.
WW1 tanks and armoured cars suffered from terrible spalling simply from machine gun fire raking the outside of the vehicle. Damaged rivet heads coming loose had the effect of ricocheting bullets inside the metal box of the crew compartment.
Spall continues to pose a problem to both designers and operators of armoured vehicles. The current trend is for a range of anti-spall liners which can be chemical and composite in nature.