Why is my iPhone 4 changing around caller ID pictures?

Recently my iPhone has started changing around the caller ID photos of my contacts. Not sure why and wondering if it might be a bug? Has anyone else had this problem and is it something to worry about?

Is it linked to Facebook? Could be updating pics when fb users do…

If the device is changing the pictures from one friend to another say? Then this is almost certainly a software problem.
I experienced this a long time ago with a iphone 4 and i had to have the handset replaced due to it having corrupted software.