Sometimes you might get internal server errors when changing your .htaccess file. A common course of these errors are invalid regular expression patterns used together with mod_rewrite.
But thats just one of the reasons, there can be many reasons why you would get 500 internal server error. Feel free to post about your own experiences with this error.
Encoding issues with htaccess
If you are using Windows, and the server you are uploading your htaccess file to is hosted on linux, then some strange things might happen with the file when its saved on your server.
Try to open the file before you upload it, and make sure that its saved in the ANSI encoding format. This can be done with a editor like Notepad++, see also: Tools list