While developing our mobile UI for SharePoint collaboration (KWizCom SharePoint Mobile Extensions) I think I stumbled upon this SharePoint bug, which to my surprise I did not notice it until now - and none of my customers ever did!
It is very simple to find, All you have to do is create a new team site, and go to Tasks list.
There you will have 2 choice fields - Status and Priority.
Both are not required, and both have default value.
Funny thing is - in edit / new item, you cannot choose empty value for these fields.
Wanna see more? Create an item and non-empty status/priority.
Next - go to these fields settings and clear the default value.
Now - you can create new items with empty status / priority, but you cannot remove the value from and item that already has a value there.
Interesting... I have to check if this still happens in 2010... :)