What's wrong with my cat command? [closed]

If he's a friend, he probably did something of the style (as root)(1):

mv /bin/cat /bin/cat.orig 
echo "echo 'cat: meow! '" > /bin/cat
chmod 755 /bin/cat

If he's less of a friend, he omitted the first line.

Asking apt-file

apt-file search -F /bin/cat 

says that the package is coreutils. You can probably reinstall it, although I see it as a bit of a dangerous thing...

(1) well, I really would have added alias which="echo cat: is a siamese" to your .bashrc, but well...

It's actually an April Fool's trick. I changed the clock on my Debian box out of April 1st, and it fixed itself. Sick, sick joke...