Someone is running a competition right now. To win, you have to find a collision using various hash functions. https://bitcointalk.org/index.php?topic=293382.0 The rules of the competition are encoded in the bitcoin transaction using Script. For example:

  OP_2DUP OP_EQUAL OP_NOT OP_VERIFY OP_SHA1 OP_SWAP OP_SHA1 OP_EQUAL

which means SHA1(x) == SHA1(y) AND NOT (x == y)