I think its quite common for various pieces of code in Sitecore to climb the item hierarchy up, seeking some inherited wisdom. A less common scenario, however, is to have a parent which is less visible than its child.

home
|
--news
   |
   --news item

(please bear with my advanced graph engine)

Imagine that you're not allowed to see the news section, but individual news items are just fine - either by mistake, or deliberately. So what happens if you have a code that goes up the tree ? (think breadcrumb)

Item item = this;
while (item != null)
{
   Collect(item.Fields["Menu Title"].Value);
   item = item.Parent;
}

You'll start with news item, which is fine. Then you go up to the news section, trying to read its field.. and get an exception, with a pretty clear message. That item that you don't have access to is considered to be "BrowseOnly", and reading fields of browse only items is bad.

To avoid seeing that exception in future, you need to add a guard clause looking for item.RuntimeSettings.BrowseOnly.

Hopefully this one stops biting me now.