Quote from: vjudeu on November 24, 2023, 10:59:20 AM

Quote

It is a synonym for OP_RETURN.

This part is wrong. A better approximation is OP_RESERVED. If you have OP_RETURN, then the Script is invalid immediately. But in case of OP_RESERVED, it can be wrapped in some unexecuted OP_IF, and it will still remain valid.

This is one of those little details in Bitcoin that not that many people know.
The only OP codes that would invalidate a script by just existing are these (like OP_CAT). However, OP codes like OP_RETURN need to be executed to invalidate a script, otherwise they can exist in an unexecuted branch for example and the script would still be valid.

Example:
https://blockstream.info/testnet/tx/95b3bccdd66d139f7f87a6c85bb704cb7278bfb8f8ff1859e3c9f99223c3bcc4?expand
The redeem script here is:

Code:

OP_IF
<push public key> OP_CheckSig
OP_ELSE
OP_RETURN
OP_ENDIF

The output was spent using the IF branch

Code:

<push signature>
OP_TRUE

Since the OP_RETURN is never reached or executed, it does no invalidate the script or the transaction. As you can see this transaction was included in testnet block #2540377