Well, Phil Karlton said that "There are only two hard things in Computer Science: cache invalidation and naming things".

Some people list off-by-one errors as the third hardest thing.

I think the joke is: "There are only two hard things in Computer Science; cache invalidation, naming things and off by one errors".

Whats the joke? The hard things are:

0) cache invalidation

1) naming things

2) off by one errors

Looks like he counted right to me.

EDIT: fixed newlines

[ "cache invalidation", "naming things", "off by one errors"].length != 2

Or clear communication and understanding of the specifications?

Seems impossible for anyone to interpret it differently than the student did, but from the comments it's clearly easy for people to extract ambiguity from what appears to be a simple, straightforward specification.