Your employment verification through ur HR or third party company will fail and they let ur hiring manager know.

You wil be in awkward position to tell everything to your immediate manager and it will be up to him to make a call. He might have to take his division director approval to over write background check result and hire anyway.

This is what usually happens in large corporate, but every setup and system is different , you might get away with all fake details without a glitch too.