Below is part of a server block for a particular vhost:

server {
server_name mysite.tld ~^.+\.mysite\.tld$;

set $base /var/www/mysite;

if ( $host ~* "^(x|y|z)\.mysite\.tld$" ){
set $files misc;
}

if ( $host ~* "^(admin)\.mysite\.tld$" ){
set $files admin;
}

if ( $host = "mysite.tld" ){
set $files home;
}

root $base/$files/www;
error_page 404 $base/errors/404.html;

location / {
try_files $uri @webapp;
}

location @webapp {
rewrite ^/([\w\d]+)/([\w\d]+)(.*)? /index.php/$1/$2?_params=$3 last;
}

location ~ \.php($|/) {
set $script $uri;
set $path_info "";

if ($uri ~ "^(.+\.php)(/.+)") {
set $script $1;
set $path_info $2;
}

fastcgi_pass unix:/tmp/php-socket;
include conf/fastcgi_params;
fastcgi_intercept_errors on;
fastcgi_param SCRIPT_FILENAME $base/$interface$script;
fastcgi_param PATH_INFO $path_info;
fastcgi_index index.php;
}

}

What I'm seeing in my error log is that the error_page its trying to
return is: /var/www/mysite/var/www/mysite/errors/404.html instead of the
expected /var/www/mysite/errors/404.html

I've also tried (with no success):

error_page 404 @notfound;
location @notfound {
root $base/errors;
error_page 404 404.html;
index 404.html;
}

Here's my version information, if it helps:

# nginx -V
nginx version: nginx/0.7.55
built by gcc 4.1.2 20080704 (Red Hat 4.1.2-44)
configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx
--conf-path=/etc/nginx/nginx.conf
--error-log-path=/var/log/nginx/error.log --pid-path=/var/run/nginx.pid
--with-rtsig_module --with-select_module --with-poll_module
--with-http_ssl_module --with-http_stub_status_module
--with-http_gzip_static_module --with-http_flv_module
--with-http_random_index_module
--http-log-path=/var/log/nginx/access.log --with-md5=/usr/lib
--with-sha1=/usr/lib --without-mail_pop3_module
--without-mail_imap_module --without-mail_smtp_module
--add-module=nginx_circle_gif-0.1.3