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.

{RPCResult::Type::NUM_TIME, "locktime", "The lock time"},
                        {RPCResult::Type::ARR, "vin", "",
                        {
                            {RPCResult::Type::OBJ, "", "",
                            {
                                {RPCResult::Type::STR_HEX, "txid", "The transaction id"},
                                {RPCResult::Type::NUM, "vout", "The output number"},
                                {RPCResult::Type::OBJ, "scriptSig", "The script",
                                {
                                    {RPCResult::Type::STR, "asm", "asm"},
                                    {RPCResult::Type::STR_HEX, "hex", "hex"},
                                }},
                                {RPCResult::Type::ARR, "txinwitness", "",
                                {
                                    {RPCResult::Type::STR_HEX, "hex", "hex-encoded witness data (if any)"},
                                }},
                                {RPCResult::Type::NUM, "sequence", "The script sequence number"},
                            }},
                        }},
                        {RPCResult::Type::ARR, "vout", "",
                        {
                            {RPCResult::Type::OBJ, "", "",
                            {
                                {RPCResult::Type::NUM, "value", "The value in " + CURRENCY_UNIT},
                                {RPCResult::Type::NUM, "n", "index"},
                                {RPCResult::Type::OBJ, "scriptPubKey", "",
                                {
                                    {RPCResult::Type::STR, "asm", "the asm"},
                                    {RPCResult::Type::STR_HEX, "hex", "the hex"},
                                    {RPCResult::Type::NUM, "reqSigs", "The required sigs"},