To Reproduce

1. set up a system with several thousand common names where the user names are all first-name-space-last-name like "jeff kirby". For example in our system we have dozens of users whose last or first name contains 'paul' and dozens of users whose last name contains 'ngu'.
2. From a page that you have full permissions on, go to Tools | Restrictions
3. Click the Person... search button to open users/userpicker.action
4. In user details, type 'paul ngu' and click search

Expected Result

The user paul ngu is at the top of the list or is the only result.

Actual Result

paul ngu doesn't appear on the list.
See

This also affects the Administration Manage Users feature.
A work around is to search by email address in the Advanced link in the userpicker but the average user, even the above average user, isn't going to do this.