Re-seasoning of cast iron skillet gone wrong? So, we had some stir-fry burn up pretty badly in our trusted cast-iron skillet. Upon cleaning the skillet some of the coating came off with the burned food. I made a first attempt of re-seasoning it with avocado oil and putting it upside down in the oven for an hour at 375°F. The result was mediocre, there was some coating in the stripped areas but it didn't look like the rest of the surface, and you could see some bare metal shining through. Sure enough, food was sticking again to the stripped area.
My wife re-seasoned it again, this time following slightly different instructions that were on the internet: using olive oil and baking it for two hours right-side-up at 450°F. At the end of the process the skillet definitely had a new hard coating that was smooth and not sticky to the touch. However, there was an anomaly in the middle (probably around the same spot that got stripped originally, but we can't be sure). There was a raised ridge around the area, but the area inside the ridge still did appear to have a proper coating:

So, my question is: what went wrong here and how does it affect the usability of the skillet? Do we need to strip it and re-season it yet again? Can we ignore this anomaly? Is there anything we can do to prevent this sort of thing from happening?
It looks like the second attempt used far too much oil, which can lead to all sorts of uneven spots in the pan. Seasoning that thick tends to flake off when you cook with the pan. I would strip it and reseason immediately. In general you want to use a very very thin layer of oil, wiping out the excess from the pan with an absorbent cloth (I don't like using paper towels since they can leave residue in the pan) before heating. There should not be enough oil in the pan to drip down the sides or form a noticeable layer in the bottom like it has done in your picture. Placing it upside down in the oven is a way of ensuring that any excess oil drips out of the pan instead of forming drips in the pan or a too thick layer at the bottom. Then you repeat the process at least three times if you start from a completely stripped pan. It's normal to require several layers before the finish is restored.