Hey all, I was just told today about an interesting wrinkle on a new feature in SharePoint 2010. As I've blogged about here before, large list throttling is a new feature that's designed to limit the impact of large lists on the health of a SharePoint farm. By default it restricts you from "impacting" more than 5000 rows of data in a single query (like a view, adding an index, etc.). The new twist on this is the scope of data that is counted in this 5000 number. It turns out that if you have deleted items from a list, all of those items still can be counted against your 5000 item limit as long as they're in the recycle bin. So how, you ask, could items in the recycle bin be used in a view on a SharePoint list? Well, they won't show up in a view, of course, but as explained here previously, you can have list throttling kick in on more than just a view of your data. For example, if you add an index, create a sorted or filtered view with a non-indexed field, etc. – that's when list throttling can be impacted by items in the recycle bin.
My good friend Tom is the one that noticed this and was able to track down the culprit, so thanks Tom for pointing this out. I'm sure this info will save some folks valuable time and hair pulling.