diff --git a/service.go b/service.go
index f611c02..e3c492d 100644
--- a/service.go
+++ b/service.go
@@ -36,6 +36,9 @@ func LoadConfig() (Config, error) {
 	if err != nil {
 		return Config{}, fmt.Errorf("invalid SYNC_INTERVAL %q: %w", raw, err)
 	}
+	if interval <= 0 {
+		return Config{}, fmt.Errorf("SYNC_INTERVAL must be positive, got %v", interval)
+	}
 	cfg.SyncInterval = interval
 
 	return cfg, nil
@@ -62,14 +65,14 @@ func NewService(cfg Config) *Service {
 	return &Service{Store: store, Syncer: syncer, interval: cfg.SyncInterval}
 }
 
-// Start loads the cached rota, runs one best-effort sync (a failure leaves the
-// cached/empty data in place — offline-tolerant), then polls in the background
-// until ctx is canceled.
+// Start loads the cached rota, runs one best-effort sync in the background
+// (a failure leaves the cached/empty data in place — offline-tolerant), then
+// polls in the background until ctx is canceled.
 func (s *Service) Start(ctx context.Context) error {
 	if err := s.Store.Load(); err != nil {
 		return err
 	}
-	_ = s.Syncer.Sync(ctx)
+	go func() { _ = s.Syncer.Sync(ctx) }()
 	go s.Syncer.Run(ctx, s.interval)
 	return nil
 }
diff --git a/service_test.go b/service_test.go
index 32df135..587bd7d 100644
--- a/service_test.go
+++ b/service_test.go
@@ -43,3 +43,11 @@ func TestLoadConfigRejectsBadInterval(t *testing.T) {
 		t.Fatal("expected error for unparseable SYNC_INTERVAL")
 	}
 }
+
+func TestLoadConfigRejectsNonPositiveInterval(t *testing.T) {
+	t.Setenv("SPREADSHEET_ID", "sheet-123")
+	t.Setenv("SYNC_INTERVAL", "0s")
+	if _, err := LoadConfig(); err == nil {
+		t.Fatal("expected error for non-positive SYNC_INTERVAL")
+	}
+}
diff --git a/store.go b/store.go
index 9083e9a..88b14bd 100644
--- a/store.go
+++ b/store.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 	"sync"
@@ -44,7 +45,8 @@ func (s *Store) Load() error {
 
 	var p persisted
 	if err := json.Unmarshal(data, &p); err != nil {
-		return fmt.Errorf("decode cache %q: %w", s.path, err)
+		log.Printf("load cache %q: corrupt, ignoring: %v", s.path, err)
+		return nil
 	}
 
 	s.mu.Lock()
@@ -95,7 +97,7 @@ func writeJSONAtomic(path string, v any) error {
 		return fmt.Errorf("create temp cache: %w", err)
 	}
 	tmpName := tmp.Name()
-	defer os.Remove(tmpName) // harmless no-op after a successful rename
+	defer func() { _ = os.Remove(tmpName) }() // harmless no-op after a successful rename
 
 	if _, err := tmp.Write(data); err != nil {
 		_ = tmp.Close()
diff --git a/store_test.go b/store_test.go
index d41669c..326b7f3 100644
--- a/store_test.go
+++ b/store_test.go
@@ -1,7 +1,9 @@
 package dyzurbot
 
 import (
+	"os"
 	"path/filepath"
+	"sync"
 	"testing"
 	"time"
 )
@@ -62,3 +64,49 @@ func TestStoreSnapshotIsACopy(t *testing.T) {
 		t.Errorf("snapshot mutation leaked into store: %q", again.Weeks[0].Person1)
 	}
 }
+
+func TestStoreLoadCorruptFileIsNotError(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "corrupt.json")
+	if err := os.WriteFile(path, []byte("{not valid json"), 0o644); err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+
+	s := NewStore(path)
+	if err := s.Load(); err != nil {
+		t.Fatalf("Load of corrupt file should be nil, got %v", err)
+	}
+	rota, ts := s.Snapshot()
+	if len(rota.Weeks) != 0 || len(rota.People) != 0 || !ts.IsZero() {
+		t.Errorf("empty store expected, got weeks=%d people=%d ts=%v", len(rota.Weeks), len(rota.People), ts)
+	}
+}
+
+func TestStoreConcurrentSnapshotAndReplace(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "rota.json")
+	s := NewStore(path)
+	var wg sync.WaitGroup
+
+	// Launch 2 goroutines repeatedly calling Replace
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < 100; j++ {
+				_ = s.Replace(sampleRota(), time.Now())
+			}
+		}()
+	}
+
+	// Launch 2 goroutines repeatedly calling Snapshot
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < 100; j++ {
+				_, _ = s.Snapshot()
+			}
+		}()
+	}
+
+	wg.Wait()
+}
diff --git a/sync.go b/sync.go
index 2b31555..ae40ced 100644
--- a/sync.go
+++ b/sync.go
@@ -33,7 +33,7 @@ func (sy *Syncer) Sync(ctx context.Context) error {
 	return sy.store.Replace(rota, sy.now())
 }
 
-// Run polls Sync on the given interval until ctx is cancelled.
+// Run polls Sync on the given interval until ctx is canceled.
 func (sy *Syncer) Run(ctx context.Context, interval time.Duration) {
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()