Whenever you cat a file, it shows nothing!
Or it destroys your terminal. 🙂
… or when you diff to files – the look identical – but it reports them as not.
You probably have control characters in the file, or no newline at the end of the file.
The example shows how to run an optical dump on a file.
# optical dump of a file, useful for seeing control chars, etc and lines with no newline.
od -c filename
[tags]Optical dump, od, Unix Coding School[/tags]