A picture of your cactus would be very helpful. The yellow spots may very well be sunburn. Although most cactus are sun plants they can sunburn when they are exposed to the sun suddenly. Just like people they need to gradually get used to lots of sun. If thats the case, the burned spots are probable there to stay. As the plant grows it may become less noticable. If the spots get mushy (rot) that is probably some disease. Overwatering can cause that. Jungle cactus, like Easter or Christmas or Epis like to be watered more frequently but most other cactus need to dry out between watering. A good moto to remember is "If you're not sure if its dry enought to water--Don't." Cacti can tolerate being too dry much better than being too wet.

I've got a Cactus with the same problem. I bought it fall of 2012, overwintered it inside under grow light and when I brought it out the next year it's growth was weird with a little "bun" growing on top. Below is a sequence of it's growth since then and a final picture of it's shape now.....Maryl Original plant Sept.2012 Next summer after bringing it out from overwintering inside under gro lights. Note top knot. Skip to 2015. Despite its rather lumpy shape it bloomed multiple times throughout the summer. Just brought it back inside for the winter a week ago. This is the shape of it now, Nov. 2015 If you look closely you can almost see the stop and start "growth rings" on the cactus. I'm thinking the change in light, location and growing conditions from hauling it in and back out affect the overall growth. I have no help for it as this is the best I can do. It's sending up a pup as we speak, so despite its rather lumpy appearance, I figure it's happy so far..........Maryl

It was the neem. Now you know, but never spray with Neem oil then put in the sun. If there's Neem oil still on the cactus, might be a good idea to get it off. Some plants don't respond well to the oily substance. I use Neem oil, but always try to do a little research on the plant I'm about to spray and how it responds to Neem. Pretty sure your cactus will be fine:). It's all a learning lesson. Goodluck:).

I am not an expert, but "weirdly skinny" usually means not enough light, so it's stretching. And the few plants I've had that turned yellow, it was because I overwatered them.