Well I don't know why but I certainly had this picture come to my mind,
Imagine a patient is admitted in the hospital for a surgery. Now he/she has 2 btc that's worth about 330 per coin in the current market. And the surgery will cost about 500$. So he/she thinks that it is enough. Now the next morning, given the nature of the Bitcoin, the price just dropped to 200 per coin. Now what will happen?
Same thing that happens if a guy only has $200 in his bank account after the surgery.
You get a bill.