Botched credit check

[octop]

Last week I signed up for Verizon's Broadband wireless plan. It turns out that they have the wrong social security number connected to my account. I have no idea how this happened (although I should have been suspicious when they made me put down a 400 dollar deposit.) I have to go to a Verizon store and straighten this out, but I was just wondering if anybody with knowledge of credit checks knows how this could have went through?