I had a problem signing up for a mobile phone contract a while ago. The mobile company eventually told me that Equifax were supplying them with (my) information which was slightly different from what I was saying about myself, so I called up Equifax. To "fix" things, Equifax wanted me to send a notarised copy of my passport to them (at my expense!)

Of course I told them to get lost and just used another mobile provider, but I learned from this episode that all of these consumer services companies share data both ways with these credit checking agencies.