1. Object-oriented programming. At that time, no one gave a clear explanation of what was actually meant by it and how to implement it. The various schools of OOP often did not acknowledge the existence of other (heretical) schools, which caused a lot of confusion in my brain. Critical voices were hounded and silenced. Also, the gap between implementation and theory was always quite wide. I did not know Smalltalk at that time and have not seen it in use to this day. 2. Electromagnetic fields. I'm still not sure I understand them. I found this video helpful: https://youtu.be/XoVW7CRR5JY