fix: changelog, check errors

Author: hstove

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to the versioning scheme outlined in the [README.md](RE
 
 ## [Unreleased]
 
+- When a transaction is dropped due to replace-by-fee, the `/drop_mempool_tx` event observer payload now includes `new_txid`, which is the transaction that replaced this dropped transaction. When a transaction is dropped for other reasons, `new_txid` is `null`. [#5381](https://github.com/stacks-network/stacks-core/pull/5381)
+
 ### Added
 - Add `tenure_timeout_secs` to the miner for determining when a time-based tenure extend should be attempted.