In 2013 Peter Todd created scripts that result in true if a hash collision is found. Bitcoin addresses resulting from these scripts can have money sent to them. If someone finds a hash collision they can spend the bitcoins on that address, so this setup acts as an incentive for somebody to do so.

For example the SHA1 script:

• scriptPubKey: OP_2DUP OP_EQUAL OP_NOT OP_VERIFY OP_SHA1 OP_SWAP OP_SHA1 OP_EQUAL
• scriptSig: <preimage1> <preimage2>

What this means:

OP_2DUP        110     0x6e    x1 x2   x1 x2 x1 x2     Duplicates the top two stack items.
OP_EQUAL       135     0x87    x1 x2   True / false    Returns 1 if the inputs are exactly equal, 0 otherwise.
OP_NOT 145     0x91    in      out     If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_VERIFY      105     0x69    True / false    Nothing / fail  Marks transaction as invalid if top stack value is not true. The top stack value is removed.
OP_SHA1        167     0xa7    in      hash    The input is hashed using SHA-1.
OP_SWAP        124     0x7c    x1 x2   x2 x1   The top two items on the stack are swapped.
OP_SHA1        167     0xa7    in      hash    The input is hashed using SHA-1.
OP_EQUAL       135     0x87    x1 x2   True / false    Returns 1 if the inputs are exactly equal, 0 otherwise.