Clients often ask for fuzzy, non-standard, maybe weird and not always logical permalink structure for their sites. If you’re now working with such a client, consider yourself lucky, because the brand new Rewrite Rules Inspector plugin is here! Props to the WordPress.com VIP team :)
There is also the Monkeyman Rewrite Analyzer – http://wordpress.org/extend/plugins/monkeyman-rewrite-analyzer/ – it allows a very detailed look at each part of a rewrite rule.