The Off-by-One Bug (The Dummy Value)

Bitcoin's multisig implementation contains one of the most famous "Bugs-turned-Consensus-Rules" in computer science. Because of an error in the original code for OP_CHECKMULTISIG, every multisig spend must include an extra, useless piece of data.

1. The Error

When Satoshi Nakamoto wrote the code for OP_CHECKMULTISIG, the function popped one too many items from the stack.

• If you have a 2-of-3 multisig, the VM expects 2 signatures and 3 keys.

• But the code logic has an "Off-by-One" error that causes it to look for 3 items to satisfy the signature requirement, even though only 2 are needed.

2. The Solution: The Dummy Value

To fix this without a hard-fork (which would have invalidated old transactions), the community decided to simply "Feed the bug."

• Every ScriptSig for a multisig spend must start with a dummy value (usually OP_0).

• The VM pops this OP_0, throws it away, and then proceeds to verify the actual signatures.

3. The Multisig Unlocking Script

A 2-of-3 multisig spend looks like this: OP_0 [Signature1] [Signature2]

If you forget the OP_0, the transaction will be rejected by the network because the VM will try to use Signature1 as the dummy value, leaving only one signature to satisfy a two-signature requirement.

4. Null Dummy Rule

To prevent miners from putting random data in this "Dummy" slot, a rule called NULLDUMMY was introduced. It requires that the dummy value must be exactly 0x00. Any other value will cause the transaction to be considered non-standard.

Stack Item	Role
OP_0	The Dummy (consumed by the bug)
Sig 1	First real signature
Sig 2	Second real signature

In the next section, we will discuss the Consensus Limits of P2MS.