Is there a continuity error between "Half Blood Prince" and "Chamber of Secrets" regarding the time the Chamber was first opened?

Question

In "Chamber of Secrets", Draco Malfoy mentions that the chamber was opened 50 years prior, and that it was before his father's time [at Hogwarts], and we know that Voldemort was the one to open it.

In the "Half Blood Prince", when inside Dumbledore's memories, we see Tom Riddle (A.K.A. Voldemort) sorted in Slytherin, and it is mentioned that Lucius Malfoy is the Prefect. That would make Lucius older than Voldemort, and since the chamber cannot have been opened before that point in time, then it couldn't have been before Lucius' time.

Is there something I am missing, or is it a continuity error the author missed?

Answer 1

I think you may be confusing things. In Harry Potter and the Deathly Hallows, we're shown that Lucius was Prefect when Snape entered Hogwarts, c.f. Chapter 33:

And Severus Snape moved off to the other side of the Hall, away from Lily, to where the Slytherins were cheering him, to where Lucius Malfoy, a prefect badge gleaming upon his chest, patted Snape on the back as he sat down beside him....

Lucius attended in 1965. Tom Riddle attended in 1937. It is possible that Abraxas, Lucius's father, was contemporary with Riddle.