In a setting I was thinking of, most of humanity lives in complex tunnel and cave systems, in sprawling masses of underground cities. To the scope of this question, let's assume their technology is comparable to the middle ages in europe. Steel is available, plate armor is available.
Most battles in this setting are fought in narrow tunnels (from 2 to 5 men wide), were armies push one against each other until one collapses.
In this hellish scenario, City X decides to use its masses of slaves in war. Those slaves would be mostly conscripted against their will (as it often is with slavery) and could serve as metaphorical cannon fodder.
The queen of city X thinks that this would be a major advantage, since the other city-states don't have similar resources of cheap, untrained muscle to flush on the war effort. Her strategy thus far is using the slave soldiers in risky engagements, and to wear out her enemies, saving the regular, well trained troops for important battles.
A peculiar style of armor is designed for the slave forces. Of course, the first thought is to send them unarmored, for cheapness sake. Yet, this is somewhat unpractical; with so little space in the tunnels, if the first rows of slaves die easily from enemy fire, the corpses can greatly slow the subsequent rows.
So, the slaves are given a one-sided armor, that covers the front part of the body only. The armor could be made of anything - metal is common in this setting, so i'm thinking plates held by leather straps over the body. The point being their back would be completely exposed, and their flanks would have minimum protection.
This would, ideally, make the slave armies less inclined to flee, since it would turn their exposed backs at the enemy. Also, a lot of emphasis could be put on discipline. It would be trivial for the slavers to hurt and punish the slave soldiers when standing behind them.