BIP141 script failure clarifications #399

  97 | @@ -98,9 +98,9 @@ If the version byte is 0, and the witness program is 32 bytes:
  98 |  * The <code>witnessScript</code> is deserialized, and executed after normal script evaluation with the remaining witness stack (≤ 520 bytes for each stack item).
  99 |  * The script must not fail, and result in exactly a single TRUE on the stack.
 100 |  
 101 | -If the version byte is 0, but the witness program is neither 20 nor 32 bytes, the script must fail.
 102 | +If the version byte is 0, but the witness program is neither 20 nor 32 bytes, the script must fail.<ref>For example, a scriptPubKey with OP_0 followed by a 40-byte non-zero data push will fail due to incorrect program size. However, a scriptPubKey with OP_0 followed by a 41-byte non-zero data push will pass, since it is not considered to be a witness program</ref>
 103 |  
 104 | -If the version byte is 1 to 16, no further interpretation of the witness program or witness happens, and there is no size restriction for the witness. These versions are reserved for future extensions.
 105 | +If the version byte is 1 to 16, no further interpretation of the witness program or witness happens, and there is no size restriction for the witness. These versions are reserved for future extensions.<ref>For backward compatibility, for any version byte from 0 to 16, the script must fail if the witness program has a <code>CastToBool</code> value of zero. However, having a hash like this is a successful preimage attack against the hash function, and the risk is negligible.</ref>

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