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.

#include <chain.h>
#include <coins.h>
#include <consensus/validation.h>
#include <core_io.h>
#include <index/txindex.h>
#include <key_io.h>
#include <merkleblock.h>
#include <node/coin.h>
#include <node/context.h>
#include <node/psbt.h>
#include <node/transaction.h>
#include <policy/policy.h>
#include <policy/rbf.h>
#include <primitives/transaction.h>
#include <psbt.h>
#include <random.h>
#include <rpc/blockchain.h>
#include <rpc/rawtransaction_util.h>
#include <rpc/server.h>
#include <rpc/util.h>
#include <script/script.h>
#include <script/sign.h>
#include <script/signingprovider.h>
#include <script/standard.h>
#include <uint256.h>
#include <util/bip32.h>
#include <util/moneystr.h>
#include <util/strencodings.h>