There are at least three instances of source code being displayed in the anime.
A teacher is seen copying some C code onto the chalkboard in Layer 01 just before Lain's fingertips begin to emit ectoplasm. It features a yen symbol instead of a backslash in an escape sequence. The teacher probably forgot something after the second percent symbol. Part of the code is not visible because the teacher is standing in the way. It reads as follows:
a='A'; b='B'; if(a>b) printf("%c>%￥n",/*teacher's head*/); if(a==b) printf("%c=% // teacher's shoulders if(a<b) printf("%c<%c // teacher's back x=321; y=123;
In Layer 07, there is a scene where Lain is in class, surreptitiously working on her HandiNAVI instead of paying attention. For a few seconds, her NAVI screen is visible, and we can see her rapidly scroll through two files of Common Lisp source code, named CRowView.CL and Afx-n-nPrv-View.CL. Text from both of the files are visible in the game as well, in the video file Dc1023, "communication."