I have just tried upgrading a blog that I help maintain to wordpress 2.5.1 and have received this error after performing the upgrade. I managed to track down the error to the query.php file inside wp-includes, but beyond that I really can't make heads or tails of the code written, partly due to not having a massive understanding of wordpress' core engine. The error I'm receiving is below, and any help that any one could provide would be greatly appreciated.
WordPress database error: [Table 'wp_posts' from one of the SELECTs cannot be used in global ORDER clause] ( SELECT SQL_CALC_FOUND_ROWS wp_posts.*, 2 AS N FROM wp_posts WHERE 1=1 AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'publish') ) UNION ALL ( SELECT wp_posts.*, 1 AS N FROM wp_posts INNER JOIN wp_postmeta ON wp_posts.ID = wp_postmeta.post_id WHERE wp_posts.post_status = 'publish' AND wp_posts.post_type = 'post' AND wp_postmeta.meta_key = 'STICKY' AND wp_postmeta.meta_value = '1' ORDER BY wp_posts.post_date_gmt DESC LIMIT 1) ORDER BY N, wp_posts.post_date DESC LIMIT 0,13