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::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", "the hex"},
                                         {RPCResult::Type::NUM, "reqSigs", "The required sigs"},
                                         {RPCResult::Type::STR, "type", "The type, eg 'pubkeyhash'"},
                                         {RPCResult::Type::ARR, "addresses", "",
                                         {
                                             {RPCResult::Type::STR, "address", "bitcoin address"},
                                         }},
                                     }},
                                 }},
                             }},
                             {RPCResult::Type::STR_HEX, "blockhash", "the block hash"},
                             {RPCResult::Type::NUM, "confirmations", "The confirmations"},
                             {RPCResult::Type::NUM_TIME, "blocktime", "The block time expressed in " + UNIX_EPOCH_TIME},
                             {RPCResult::Type::NUM, "time", "Same as \"blocktime\""},
                        }
                    },
                },
                RPCExamples{
                    HelpExampleCli("getrawtransaction", "\"mytxid\"")
            + HelpExampleCli("getrawtransaction", "\"mytxid\" true")
            + HelpExampleRpc("getrawtransaction", "\"mytxid\", true")