diff --git a/src/adapter/src/client.rs b/src/adapter/src/client.rs
index 0405a7b0a7084..7c8b15ebd9276 100644
--- a/src/adapter/src/client.rs
+++ b/src/adapter/src/client.rs
@@ -624,18 +624,25 @@ impl SessionClient {
         result
     }
 
-    /// Ends a transaction.
+    /// Ends a transaction. Even if an error is returned, guarantees that the transaction in the
+    /// session and Coordinator has cleared its state.
     #[instrument(level = "debug")]
     pub async fn end_transaction(
         &mut self,
         action: EndTransactionAction,
     ) -> Result<ExecuteResponse, AdapterError> {
-        self.send(|tx, session| Command::Commit {
-            action,
-            session,
-            tx,
-        })
-        .await
+        let res = self
+            .send(|tx, session| Command::Commit {
+                action,
+                session,
+                tx,
+            })
+            .await;
+        // Commit isn't guaranteed to set the session's state to anything specific, so clear it
+        // here. It's safe to ignore the returned `TransactionStatus` because that doesn't contain
+        // any data that the Coordinator must act on for correctness.
+        let _ = self.session().clear_transaction();
+        res
     }
 
     /// Fails a transaction.
diff --git a/src/adapter/src/command.rs b/src/adapter/src/command.rs
index 2e4eeb847f968..08f2c10698459 100644
--- a/src/adapter/src/command.rs
+++ b/src/adapter/src/command.rs
@@ -71,6 +71,10 @@ pub enum Command {
         outer_ctx_extra: Option<ExecuteContextExtra>,
     },
 
+    /// Attempts to commit or abort the session's transaction. Guarantees that the Coordinator's
+    /// transaction state has been cleared, even if the commit or abort fails. (A failure can
+    /// happen, for example, if the session's role id has been dropped which will prevent
+    /// sequence_end_transaction from running.)
     Commit {
         action: EndTransactionAction,
         session: Session,
diff --git a/src/adapter/src/coord/command_handler.rs b/src/adapter/src/coord/command_handler.rs
index 74956ec147677..78381a4e5982c 100644
--- a/src/adapter/src/coord/command_handler.rs
+++ b/src/adapter/src/coord/command_handler.rs
@@ -22,9 +22,9 @@ use futures::FutureExt;
 use mz_adapter_types::connection::{ConnectionId, ConnectionIdType};
 use mz_catalog::memory::objects::{CatalogItem, DataSourceDesc, Source};
 use mz_catalog::SYSTEM_CONN_ID;
-use mz_ore::instrument;
 use mz_ore::task;
 use mz_ore::tracing::OpenTelemetryContext;
+use mz_ore::{instrument, soft_panic_or_log};
 use mz_repr::role_id::RoleId;
 use mz_repr::{ScalarType, Timestamp};
 use mz_sql::ast::{
@@ -215,8 +215,12 @@ impl Coordinator {
                         }
                     };
 
+                    let conn_id = ctx.session().conn_id().clone();
                     self.sequence_plan(ctx, plan, ResolvedIds(BTreeSet::new()))
                         .await;
+                    // Part of the Command::Commit contract is that the Coordinator guarantees that
+                    // it has cleared its transaction state for the connection.
+                    self.clear_connection(&conn_id).await;
                 }
 
                 Command::CatalogSnapshot { tx } => {
@@ -749,9 +753,11 @@ impl Coordinator {
                     // (Terminating the connection is maybe what we would prefer to do, but is not
                     // currently a thing we can do from the coordinator: calling handle_terminate
                     // cleans up Coordinator state for the session but doesn't inform the
-                    // AdapterClient that the session should terminate.) Once the bug here
-                    // (https://github.com/MaterializeInc/materialize/issues/28400) is fixed, this
-                    // could be changed to an assert.
+                    // AdapterClient that the session should terminate.)
+                    soft_panic_or_log!(
+                        "session {} attempted to get ddl lock while already owning it",
+                        ctx.session().conn_id()
+                    );
                     ctx.retire(Err(AdapterError::Internal(format!(
                         "session attempted to get ddl lock while already owning it"
                     ))));
diff --git a/src/adapter/src/coord/sql.rs b/src/adapter/src/coord/sql.rs
index 0ddeb6edbd887..e0ec8f8f87892 100644
--- a/src/adapter/src/coord/sql.rs
+++ b/src/adapter/src/coord/sql.rs
@@ -192,6 +192,12 @@ impl Coordinator {
         &mut self,
         session: &mut Session,
     ) -> TransactionStatus<mz_repr::Timestamp> {
+        // This function is *usually* called when transactions end, but it can fail to be called in
+        // some cases (for example if the session's role id was dropped, then we return early and
+        // don't go through the normal sequence_end_transaction path). The `Command::Commit` handler
+        // and `AdapterClient::end_transaction` protect against this by each executing their parts
+        // of this function. Thus, if this function changes, ensure that the changes are propogated
+        // to either of those components.
         self.clear_connection(session.conn_id()).await;
         session.clear_transaction()
     }