What is going on with this leaf?

just a mutation. shouldn't be a problem. sometimes a tiny bud can grow between petiole and leaf, too.

Quite common and nothing is wrong. I have grown plants that grow flowers in that same area, flowers on leaf stems, flowers out of the main stem, two leaves per node, branches dividing into multiple branches etc etc...........you name it...........cannabis can do some amazing things at times!

common mutation, nothing special

Nur eine Mutation. Das gibt es oft. Und manchen sind richtig skurril :)

I a little leaf on a leaf. Mutation, nothing to crazy IMO. I have seen mini buds grow out of that same area on some plants I had before its fun. I don't think its anything to worry about. Sometimes the colour of the stem can indicate issues with P or Mg but LED lights also bring out weird colours in stems and some plats are more prone to being red/purple.

Just double leaf mutation, usually you will have small bud there if you will keep it till the end of flower.