While trying to install WordPress MU, I had a 500 Internal Server Error.
This was tested on Apache 1.3 on a Windows XP box, so it may not work for you. Anyways, this is what I did to remove the error:
Edit .htaccess:
1) Changed RewriteBase \/ to RewriteBase /
2) At the bottom, under the comment '#catch all rules':
Original:
RewriteCond c:/wordpress/wp-inst/wp-content/blogs/$1 -d
RewriteRule ^([_0-9a-z-]+)/ /wp-inst/index.php [L]
Modified (added double quotes)
RewriteCond "c:/wordpress/wp-inst/wp-content/blogs/$1" -d
RewriteRule ^([_0-9a-z-]+)/ /wp-inst/index.php [L]
3) I added double quotes for the second last line in .htaccess too.
I'm not sure if these errors are specific to Windows only, though I suspect they are.
I hope this will be of use to you.