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.

keypaths.push_back(keypath);
            }
            out.pushKV("bip32_derivs", keypaths);
        }
        
        if (output.unknown.size() > 0) {
            UniValue unknowns(UniValue::VOBJ);
            for (auto entry : output.unknown) {
                unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));
            }
            out.pushKV("unknown", unknowns);
        }
        outputs.push_back(out);
        
        if (MoneyRange(psbtx.tx->vout[i].nValue) && MoneyRange(output_value + psbtx.tx->vout[i].nValue)) {
            output_value += psbtx.tx->vout[i].nValue;
        } else {
            
            have_all_utxos = false;
        }
    }
    result.pushKV("outputs", outputs);
    if (have_all_utxos) {
        result.pushKV("fee", ValueFromAmount(total_in - output_value));
    }
    return result;
},
    };
}
static RPCHelpMan combinepsbt()