Sometimes Sitecore will not return certain items. For instance, in the following code:
Sitecore.Data.ID itemId = new Sitecore.Data.ID(id);
Sitecore.Context.Database.GetItem(id);
This will return null, however if the same string id is placed on the search above the content tree in the content editor, it will return the item! The context database is web and I've republished the item several times.
I am using Sitecore 6.2.0 (rev. 100701).