Brown Scarring on Cactus Due to Surface Damage

Damage to cow's tongue, Opuntia

Cochineal scale on cows tongue

Q. Not sure what happened to my plant I sent you a picture of this plant the other day and the next day I see these spots. All I did was remove a chicken wire fence from around the plant.

A. I would not be overly concerned about it from a plant health perspective. Yes, it does distract from its aesthetics. This type of injury can because by something damaging the surface of the pads at those locations.
 

A frequent problem can be cochineal scale which appears like wet cotton on the surface of the pads in groupings. Once the scale is removed either with a high pressure sweep nozzle on a hose or chemicals the damage can still remain from their feeding. But anything that damages the "skin" of the pad will cause this type of scarring.

Make sure that you are not irrigating this type of cactus too often or they can develop root rot and die. 

Also, if you are using drip emitters and they are all located very close to the plant and there are no other sources of water than the roots may develop only in this tiny location and when it gets tall it can fall over because of its own weight, it's top-heavy. So hopefully there are other sources of water within a foot or two of this plant so the roots will spread out and hold the plant upright.