Dijkstra had a unique format for his undergraduate class where the entire grade basically came down to an interview at the end of the semester. During which he asked you to work out a solution to a problem in front of him one on one. A friend had the most memorable interaction with him during the interview/final exam. Dijkstra explained the problem and they began furiously writing out a solution in pencil getting a few lines into the proof before realizing they had made a mistake and began to erase what was written. Dijkstra responded "tsk tsk tsk, you are a rash mathematician" and advised the student to "Use a pen instead of pencil it will encourage you to spend more time with your thoughts before writing."

A couple years later I saw a quote that reminded me of that interaction and it goes “I mean, if 10 years from now, when you are doing something quick and dirty, you suddenly visualize that I am looking over your shoulders and say to yourself "Dijkstra would not have liked this", well, that would be enough immortality for me.”

Dijkstra's ghost haunts us all.