A female robot caste that is just a squishy round ball, if someone decides it wants a sphere for a wife, it will spit out a ridiculous amount of babies.
Let's say more advanced weapon or armormaking has a chance of releasing a lethal toxin when forging, and make a robot caste that learns to forge metals fairly quickly, and is immune to said toxin, it cannot learn to fight easily, if at all, but is still rather hardy and is equipped with a forging hammer arm.
A medical spiderdroid-thingy that begins with some skill in medicine and learns said skills quickly, has a bonesaw bodypart, among other things, but is even worse at combat then the forgerbot, and is quite brittle, but also fast, It'd be cool if it injected various poisons into enemies if you put one in your military.
And.. a spherical droid, VERY good observing, made up of a fragile and possibly expensive 'eye' and a thin copper shell, it moves faster then the medical spider but has absolutely no attacking ability except for pushing, it's meant to be posted at your fort's entrance with restraints to find the skulking filth, so this one could be a pet, obviously trainable into a war animal, you know, if you want fast moving spheres to throw themselves at enemies.
I don't recommend taking the first suggestion seriously at all.