I thought some newcomers might find an explanation of these various errors useful, especially dealing with complex stuff like rewrite directives(which I am just learning myself).
Please feel free to add your thoughts on these, and correct me if I am wrong.
403: Forbidden. This most likely means you(or the script you are trying to run) does not have permissions to the file or directory. Using the chmod command, in your FTP client or locally, try setting the file or directory you are trying to access to: 777 which means read, write, execute for all users. This is just to try and narrow down the problem however. MAKE SURE YOU CHANGE THIS AFTER. Something like 644 (read, write for the owner, read only for everyone else) should be sufficient for file permissions, and 755 for directories.
404: The well known 'file not found' error. You might get this while trying to enable permalinks. I have found that it is most likely due to having the wrong directory listed as your Rewrite Base in .htaccess. WordPress says to use / but sometimes this is incorrect. For example on my server I need to use: Rewrite Base /home/adailyad/public_html/ Either call your ISP or just keep trying different paths till you get it. Running phpinfo.php (in your wordpress directory) might give you some hints, look for the path value.
500: This is a internal server error. Most likely caused by a mistake in your .htaccess file. It's basically the server's way of telling you that you tried something you aren't allowed to do. Many people have trouble with mod_rewrite while trying to enable permalinks. Or you might have made a mistake writing an access rule.
If you are trying to turn on the Rewrite engine, and keep getting this error try this in your .htaccess file:
Notice the <IFModule mod_rewrite.c> and the </IfModule> commands. These are not given when WordPress provides the code for permalinks. Add your directives and commands after the Rewrite Base and before the </IfModule>
Hope this helps and Good luck!