An ErrorDocument for code 200 would cause an infinite loop,

http://www.javascriptkit.com/howto/htaccess2.shtml

Error Documents

This seems to be what people think htaccess was meant for, but it is only part of the general use. We'll be getting into progressively more advanced stuff after this.

Successful Client Requests
200	OK
201	Created
202	Accepted
203	Non-Authorative Information
204	No Content
205	Reset Content
206	Partial Content
Client Request Redirected
300	Multiple Choices
301	Moved Permanently
302	Moved Temporarily
303	See Other
304	Not Modified
305	Use Proxy
Client Request Errors
400	Bad Request
401	Authorization Required
402	Payment Required (not used yet)
403	Forbidden
404	Not Found
405	Method Not Allowed
406	Not Acceptable (encoding)
407	Proxy Authentication Required
408	Request Timed Out
409	Conflicting Request
410	Gone
411	Content Length Required
412	Precondition Failed
413	Request Entity Too Long
414	Request URI Too Long
415	Unsupported Media Type
Server Errors
500	Internal Server Error
501	Not Implemented
502	Bad Gateway
503	Service Unavailable
504	Gateway Timeout
505	HTTP Version Not Supported

In order to specify your own ErrorDocuments, you need to be slightly familiar with the serverreturned error codes. (List to the right). You do not need to specify error pages for all of these, in fact you shouldn't. An ErrorDocument for code 200 would cause an infinite loop, whenever a page was found...this would not be good.You will probably want to create an error document for codes 404 and 500, at the least 404 since this would give you a chance to handle requests for pages not found. 500 would help you out with internal server errors in any scripts you have running. You may also want to consider ErrorDocuments for 401 - Authorization Required (as in when somebody tries to enter a protected area of your site without the proper credentials), 403 - Forbidden (as in when a file with permissions not allowing it to be accessed by the user is requested) and 400 - Bad Request, which is one of those generic kind of errors that people get to by doing some weird stuff with your URL or scripts.
In order to specify your own customized error documents, you simply need to add the following command, on one line, within your htaccess file:

ErrorDocument code /directory/filename.ext
or
ErrorDocument 404 /errors/notfound.html
This would cause any error code resulting in 404 to be forward to yoursite.com/errors/notfound.html

Likewise with:
ErrorDocument 500 /errors/internalerror.html

You can name the pages anything you want (I'd recommend something that would prevent you from forgetting what the page is being used for), and you can place the error pages anywhere you want within your site, so long as they are web-accessible (through a URL). The initial slash in the directory location represents the root directory of your site, that being where your default page for your first-level domain is located. I typically prefer to keep them in a separate directory for maintenance purposes and in order to better control spiders indexing them through a ROBOTS.TXT file, but it is entirely up to you.
If you were to use an error document handler for each of the error codes I mentioned, the htaccess file would look like the following (note each command is on its own line):

ErrorDocument 400 /errors/badrequest.html
ErrorDocument 401 /errors/authreqd.html
ErrorDocument 403 /errors/forbid.html
ErrorDocument 404 /errors/notfound.html
ErrorDocument 500 /errors/serverr.html

You can specify a full URL rather than a virtual URL in the ErrorDocument string (http://yoursite.com/errors/notfound.html vs. /errors/notfound.html). But this is not the preferred method by the server's happiness standards.
You can also specify HTML, believe it or not!

ErrorDocument 401 "
You have
 to actually BE a member to view 
this page, Colonel!

The only time I use that HTML option is if I am feeling particularly saucy, since you can have so much more control over the error pages when used in conjunction with xSSI or CGI or both. Also note that the ErrorDocument starts with a " just before the HTML starts, but does not end with one...it shouldn't end with one and if you do use that option, keep it that way. And again, that should all be on one line, no naughty word wrapping!
Next, we are moving on to password protection, that last frontier before I dunk you into the true capabilities of htaccess. If you are familiar with setting up your own password protected directories via htaccess, you may feel like skipping ahead.


-- map{ map{tr|10|# |;print} split//,sprintf"%.8b\n",$_} unpack'C*',unpack'u*',"5`#8<3'X`'#8^-@`<-CPP`#8V/C8`"