$base redefined?

I have $base defined as blog_demo, yet for some commands WP is defining it as simply “demo” and (obviously) not finding the database. It’s giving me errors like this:
Database error: [Table ‘demo.wp_comments’ doesn’t exist]
SELECT * FROM wp_comments WHERE comment_post_ID = ‘3’ AND comment_approved = ‘1’ ORDER BY comment_date
The weird part is that it only does this for some things — for other commands (like, for example, displaying posts), it gets the database just right.