P2PKH and P2SH outputs where the embedded hash push length is not 20 bytes are permanently unspendable, because OP_HASH160 always produces exactly 20 bytes and the comparison can never succeed.

Abstract

P2PKH

Standard P2PKH execution: the spending scriptSig pushes a pubkey and signature; the script then executes OP_DUP OP_HASH160 to hash the pubkey to exactly 20 bytes, then OP_EQUALVERIFY compares the hash to the pushed value. If the pushed value is not 20 bytes, the comparison of differently-sized values always fails — the output is unspendable regardless of the spending input.

P2SH

Standard P2SH execution ends with OP_HASH160 <push> OP_EQUAL. OP_HASH160 always returns 20 bytes. If <push> is not 20 bytes, OP_EQUAL always returns false and the spend fails.

Note: Proposal 003 (MtGox) already handles the specific zero-length case 76a90088ac. This proposal covers all other non-20-byte lengths (1–19 and 21–75 bytes).

Script Formats

P2PKH: 76 a9 [push-byte] [data] 88 ac

• 76 = OP_DUP, a9 = OP_HASH160, 88 = OP_EQUALVERIFY, ac = OP_CHECKSIG
• Standard: push-byte = 14 (20 bytes)

P2SH: a9 [push-byte] [data] 87

• a9 = OP_HASH160, 87 = OP_EQUAL
• Standard: push-byte = 14 (20 bytes)

Detection

For each format, parse the push byte and flag if it is a direct push (≤ 0x4b), the script length is consistent with the declared length, and the push length is not 20.

// P2PKH: 76 a9 [push] [data] 88 ac
if (hex.startsWith('76a9') && hex.endsWith('88ac') && hex.length >= 10) {
    const pushByte = parseInt(hex.slice(4, 6), 16);
    if (pushByte <= 0x4b &&
        hex.length === 6 + pushByte * 2 + 4 &&
        pushByte !== 20) {
        return true;
    }
}
// P2SH: a9 [push] [data] 87
else if (hex.startsWith('a9') && hex.endsWith('87') && hex.length >= 8) {
    const pushByte = parseInt(hex.slice(2, 4), 16);
    if (pushByte <= 0x4b &&
        hex.length === 4 + pushByte * 2 + 2 &&
        pushByte !== 20) {
        return true;
    }
}

Scope Notes

• Only direct push opcodes (≤ 0x4b) are checked. OP_PUSHDATA variants could theoretically be used but would produce non-standard scripts that are also unspendable for other reasons.
• Proposal 003 (MtGox zero-byte hash) is evaluated before this proposal and catches 76a90088ac, so there is no double-counting.