A case in which cat(1) program is used, even when it isn't technically needed.
A classic case is filtering. Usually, filters made with Perl can take their stuff from stdin OR get the stuff from text file given as command-line parameter.
Also, < shell operator can be used to achieve the same effect without invoking cat. In general, shell piping / redirection can be used in almost all cases, and cat is needed in surprisingly few cases.
$ cat stuff.txt | filter > results.txt
can be avoided by using either:
$ filter stuff.txt > results.txt
$ filter < stuff.txt > results.txt
In some newsgroups (comp.unix.shell, I've heard) there's a tradition of handing out Useless Use of Cat Awards for worst abuses of redundant usage of cat and other shell idioms. This tradition was started by Perl guru Randal Schwartz.