[tsimionescu]

Let's say you just sent someone 1000 dollars in exchange for 1 Eth, sometime close to the fork. Before the transaction, you have 1000 USD and 0 Eth. In the old block chain, the transaction goes through, your wallet has 1 Eth, and theirs has 0. In the forked block chain, their wallet has 1 Eth, your has 0. However, regardless of which block chain you choose to believe in, they have the 1000 dollars.

So if the forked chain becomes accepted, then they have 1000 USD and 1 Eth, while you have 0 dollars and 0 Eth, and you're never going to make the mistake of trusting a cryptocurrency ever again.