diff --git a/clientapi/routing/register.go b/clientapi/routing/register.go
index 6827bed22..8faff58f8 100644
--- a/clientapi/routing/register.go
+++ b/clientapi/routing/register.go
@@ -156,6 +156,13 @@ func (d *sessionsDict) startTimer(duration time.Duration, sessionID string) {
 	})
 }
 
+func (d *sessionsDict) hasSession(sessionID string) bool {
+	d.RLock()
+	defer d.RUnlock()
+	_, ok := d.sessions[sessionID]
+	return ok
+}
+
 // addCompletedSessionStage records that a session has completed an auth stage
 // also starts a timer to delete the session once done.
 func (d *sessionsDict) addCompletedSessionStage(sessionID string, stage authtypes.LoginType) {
diff --git a/clientapi/routing/register_publickey.go b/clientapi/routing/register_publickey.go
index 258a47249..f5c972bb1 100644
--- a/clientapi/routing/register_publickey.go
+++ b/clientapi/routing/register_publickey.go
@@ -62,7 +62,7 @@ func handlePublicKeyRegistration(
 		return false, authtypes.LoginStagePublicKeyNewRegistration, nil
 	}
 
-	if _, ok := sessions.sessions[authHandler.GetSession()]; !ok {
+	if !sessions.hasSession(authHandler.GetSession()) {
 		return false, "", &util.JSONResponse{
 			Code: http.StatusUnauthorized,
 			JSON: jsonerror.Unknown("the session ID is missing or unknown."),
diff --git a/roomserver/internal/perform/perform_backfill.go b/roomserver/internal/perform/perform_backfill.go
index 57e121ea2..7d65c44fb 100644
--- a/roomserver/internal/perform/perform_backfill.go
+++ b/roomserver/internal/perform/perform_backfill.go
@@ -18,6 +18,7 @@ import (
 	"context"
 	"fmt"
 
+	"github.com/getsentry/sentry-go"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 	"github.com/sirupsen/logrus"
@@ -322,6 +323,7 @@ FederationHit:
 		b.eventIDToBeforeStateIDs[targetEvent.EventID()] = res
 		return res, nil
 	}
+	sentry.CaptureException(lastErr) // temporary to see if we might need to raise the server limit
 	return nil, lastErr
 }