Occasionally, you might stumble upon a strange error, that happens both on production servers and dev server (ie with the php dev server). You might try to edit (enable/disable/reorder) a menu item in a Drupal menu, but this won't be saved.
This could happen on really big menus, with a lot of items in them. PHP is blocking this change, as a security measure!
Slow query logs are a powerfull tool to monitor your sites bottlenecks. However, unless you wanna be working with old data, you need to rotate them to see if you are actually progressing or not.
When working on sites with thousands or millions of users you have to face a very important problem: your test/dev sites are not allowed to send emails, at all! Develmail and other solutions exist but most of them are very limiting because you end up with text emails and not your nicely styled HTML mail. A way to overcome this problem is by using an external SMTP server that you manage. I'm using iRedmail which works with Postfix.
