Here is possible attack scenario: 1. Alice allows Bob to transfer N of Alice's tokens (N>0) by calling approve method on Token smart contract passing Bob's address and N as method arguments 2. After some time, Alice decides to change from N to M (M>0) the number of Alice's tokens Bob is allowed to transfer, so she calls approve method again, this time passing Bob's address and M as method arguments 3. Bob notices Alice's second transaction before it was mined and quickly sends another transaction that calls transferFrom method to transfer N Alice's tokens somewhere 4. If Bob's transaction will be executed before Alice's transaction, then Bob will successfully transfer N Alice's tokens and will gain an ability to transfer another M tokens 5. Before Alice noticed that something went wrong, Bob calls transferFrom method again, this time to transfer M Alice's tokens.
So, Alice's attempt to change Bob's allowance from N to M (N>0 and M>0) made it possible for Bob to transfer N+M of Alice's tokens, while Alice never wanted to allow so many of her tokens to be transferred by Bob.