To add to Glorfindel's answer:
Randall Munroe once made a comic about a programming error that was very common decades ago, which allowed hackers to have gain control over databases even if they did not have administrator credentials. When a programmer made this mistake, we could usually modify data as much as we wanted, delete whole databases, or bring a site down by overwhelming its hardware with useless queries.
In the comic, a woman named her son in such a way that inputting his name in an electronic system would delete student data (should the system have a table called Students). Some school is calling the mother to complain about what she did, and she replies by letting the school know how amateurish their IT work is.
Notice that this hotlinking of the image above is allowed by the author. Check the source link above.
Since it was a very common mistake, programmers went en masse to Stack Overflow (and other sites not in Stack Exchange) for a solution. There was an absurd amount of duplicate questions - so we were joking that reading that specific XKCD comic was part of the mandatory education for a programmer.
Back then Stack Exchange did not have many sites as it does today. Besides SO there was just Super User and Server Fault, if I remember correctly. Those were all aimed at IT, so references to XKCD were mostly about that comic or some other related ones.
Since a lot of Worldbuilding users are also SO users, the tradition made its way here memetically, but the "mandatory" part of the joke lost its original meaning. Now it is said as if speaking of a subject causes an obligation to reference a related XKCD comic should one exist, not that the specific comic contains mandatory knowledge for someone to work in a field.
On a not unrelated note, I feel really old now.