I once screwed up my neem oil application, and it's pretty obvious that something is wrong once it happens.
Usually I'll dip my smaller plants in a mixture of insecticidal soap and water, but I was all out of soap and so I used some neem oil.
Turns out, I didn't mix the oil in the water very well, and the oil was resting on top of the water (as oil does) so when I dipped the plant, it was pretty much covered in neem oil. The leaves turned kind of a translucent, browny, yellow, and became very crispy.
Needless to say, the plant died.
I wish I had taken some pictures, but I, like most people, tend to take pictures of my successes rather than my failures.