diff --git a/pkg/store/cache.go b/pkg/store/cache.go
index 16cad121f5..5acd2c104c 100644
--- a/pkg/store/cache.go
+++ b/pkg/store/cache.go
@@ -43,6 +43,7 @@ type indexCache struct {
 	added       *prometheus.CounterVec
 	current     *prometheus.GaugeVec
 	currentSize *prometheus.GaugeVec
+	overflow    *prometheus.CounterVec
 }
 
 // newIndexCache creates a new LRU cache for index entries and ensures the total cache
@@ -66,6 +67,11 @@ func newIndexCache(reg prometheus.Registerer, maxBytes uint64) (*indexCache, err
 		Help: "Total number of requests to the cache.",
 	}, []string{"item_type"})
 
+	c.overflow = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Name: "thanos_store_index_cache_items_overflowed_total",
+		Help: "Total number of items that could not be added to the cache due to being too big.",
+	}, []string{"item_type"})
+
 	c.hits = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Name: "thanos_store_index_cache_hits_total",
 		Help: "Total number of requests to the cache that were a hit.",
@@ -115,10 +121,16 @@ func newIndexCache(reg prometheus.Registerer, maxBytes uint64) (*indexCache, err
 	return c, nil
 }
 
-func (c *indexCache) ensureFits(b []byte) {
+// ensureFits tries to make sure that the passed slice will fit into the LRU cache.
+// Returns true if it will fit.
+func (c *indexCache) ensureFits(b []byte) bool {
+	if uint64(len(b)) > c.maxSize {
+		return false
+	}
 	for c.curSize+uint64(len(b)) > c.maxSize {
 		c.lru.RemoveOldest()
 	}
+	return true
 }
 
 func (c *indexCache) setPostings(b ulid.ULID, l labels.Label, v []byte) {
@@ -127,7 +139,10 @@ func (c *indexCache) setPostings(b ulid.ULID, l labels.Label, v []byte) {
 	c.mtx.Lock()
 	defer c.mtx.Unlock()
 
-	c.ensureFits(v)
+	if !c.ensureFits(v) {
+		c.overflow.WithLabelValues(cacheTypePostings).Inc()
+		return
+	}
 
 	// The caller may be passing in a sub-slice of a huge array. Copy the data
 	// to ensure we don't waste huge amounts of space for something small.
@@ -158,7 +173,10 @@ func (c *indexCache) setSeries(b ulid.ULID, id uint64, v []byte) {
 	c.mtx.Lock()
 	defer c.mtx.Unlock()
 
-	c.ensureFits(v)
+	if !c.ensureFits(v) {
+		c.overflow.WithLabelValues(cacheTypeSeries).Inc()
+		return
+	}
 
 	// The caller may be passing in a sub-slice of a huge array. Copy the data
 	// to ensure we don't waste huge amounts of space for something small.
diff --git a/pkg/store/cache_test.go b/pkg/store/cache_test.go
new file mode 100644
index 0000000000..2846513a0f
--- /dev/null
+++ b/pkg/store/cache_test.go
@@ -0,0 +1,22 @@
+// Tests out the index cache implementation.
+package store
+
+import (
+	"testing"
+
+	"github.com/improbable-eng/thanos/pkg/testutil"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// TestIndexCacheEdge tests the index cache edge cases.
+func TestIndexCacheEdge(t *testing.T) {
+	metrics := prometheus.NewRegistry()
+	cache, err := newIndexCache(metrics, 1)
+	testutil.Ok(t, err)
+
+	fits := cache.ensureFits([]byte{42, 24})
+	testutil.Equals(t, fits, false)
+
+	fits = cache.ensureFits([]byte{42})
+	testutil.Equals(t, fits, true)
+}