Nobody forgets a compilers course.
A "compilers" course is probably the first CS course where debugging is real--and terrifyingly hard. A compilers course converts a programmer from a "null pointers are bad" person to a "null pointers are evil" zealot.
Compilers courses are probably the closest that a CS person will get to PTSD.
I've taken a compilers course, at Stanford no less.
I've written a couple of compilers and some near-compilers. (A very fast simulator can be a lot like a compiler.)
I don't think that null pointers are evil. In fact, I like null pointers.
My beef with null pointers is that very few languages support multiple null pointers. (No, mocked objects or sentinels are NOT a good substitute.)
Yes, I want multiple isNull values that can be distinguished but can't be used. (Python's "lots of things are false and you can create more" is arguably the closest.)
I've seen it available on Coursera. And a lot of the current material is available online. But I tried the Coursera one awhile ago and got stuck.
I'm finishing a CS degree but there's no dedicated compiler course here.