Imagine you have a landlord who collects your rent check and deposits it in a very unpredictable way. Sometimes he does it immediately. Sometimes he collects the checks for months before cashing them.

Initially you feel happy about this system because you get to live in your apartment without the money actually leaving your bank account. But soon you realize that the extra money is just a mirage. You can't plan how much money you need to have at any given time because your landlord may cash your checks right when you don't have the balance to back it.

This is the danger of lazy evaluation.