I don't need to do this often but occasionally you'll need to fix inconsistent line endings, change encoding to UTF-8 or remove a BOM [0] (to stop  being prefixed by some old software).
In OS X and Linux (and Windows with Cygwin etc) there are several command line programs such as unix2dos (line ending conversion), iconv (encoding conversions) etc for doing just that, so that you don't need to do it in an editor and can even automate the work.
[0] https://en.wikipedia.org/wiki/Byte_order_mark