I discovered a race condition in sendrawtransaction. For details: see my comment in https://github.com/bitcoin/bitcoin/commit/1d46fe3327f6645fd79b442cd72ef422418c1a50.

If you send a raw transaction which has already been broadcasted, then you don't know whether you'll receive an RPC_TRANSACTION_ALREADY_IN_CHAIN exception, since you don't know whether a new block has just been received, which includes the transaction. There are use cases where such RPC_TRANSACTION_ALREADY_IN_CHAIN occurrences do not indicate an error; however, since this type of occurrence is very rare, developers are not likely to be aware of the possibility, and silence the exception. As a result, applications can potentially experience hard-to-diagnose, very rare errors.

I think the core of the problem is that sendrawtransaction tries to combine two use cases:
A: publish a transaction on the Bitcoin network
B: re-transmit an already known transaction to peers
RPC_TRANSACTION_ALREADY_IN_CHAIN might make sense for B, but for A (which is, IMHO, the primary use case) it is confusing.

rbf = request.params[3].isTrue();
    }
    CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], rbf);
    return EncodeHexTx(CTransaction(rawTx));
},
    };
}
static RPCHelpMan decoderawtransaction()
{
    return RPCHelpMan{"decoderawtransaction",
                "\nReturn a JSON object representing the serialized, hex-encoded transaction.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction hex string"},
                    {"iswitness", RPCArg::Type::BOOL,  "depends on heuristic tests", "Whether the transaction hex is a serialized witness transaction.\n"
                        "If iswitness is not present, heuristic tests will be used in decoding.\n"
                        "If true, only witness deserialization will be tried.\n"
                        "If false, only non-witness deserialization will be tried.\n"
                        "This boolean should reflect whether the transaction has inputs\n"
                        "(e.g. fully valid, or on-chain transactions), if known by the caller."
                    },
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR_HEX, "txid", "The transaction id"},
                        {RPCResult::Type::STR_HEX, "hash", "The transaction hash (differs from txid for witness transactions)"},
                        {RPCResult::Type::NUM, "size", "The transaction size"},
                        {RPCResult::Type::NUM, "vsize", "The virtual transaction size (differs from size for witness transactions)"},
                        {RPCResult::Type::NUM, "weight", "The transaction's weight (between vsize*4 - 3 and vsize*4)"},
                        {RPCResult::Type::NUM, "version", "The version"},