diff --git a/Lib/test/test_zoneinfo/test_zoneinfo.py b/Lib/test/test_zoneinfo/test_zoneinfo.py
index 85877c984c8..e9516c0b127 100644
--- a/Lib/test/test_zoneinfo/test_zoneinfo.py
+++ b/Lib/test/test_zoneinfo/test_zoneinfo.py
@@ -741,6 +741,38 @@ def test_empty_zone(self):
         with self.assertRaises(ValueError):
             self.klass.from_file(zf)
 
+    def test_invalid_transition_index(self):
+        STD = ZoneOffset("STD", ZERO)
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+
+        zf = self.construct_zone([
+            ZoneTransition(datetime(2026, 3, 1, 2), STD, DST),
+            ZoneTransition(datetime(2026, 11, 1, 2), DST, STD),
+        ], after="", version=1)
+
+        data = bytearray(zf.read())
+        timecnt = struct.unpack_from(">l", data, 32)[0]
+        idx_offset = 44 + timecnt * 4
+        data[idx_offset + 1] = 2  # typecnt is 2, so index 2 is OOB
+        f = io.BytesIO(bytes(data))
+
+        with self.assertRaises(ValueError):
+            self.klass.from_file(f)
+
+    def test_transition_lookahead_out_of_bounds(self):
+        STD = ZoneOffset("STD", ZERO)
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+        EXT = ZoneOffset("EXT", ONE_H)
+
+        zf = self.construct_zone([
+            ZoneTransition(datetime(2026, 3, 1), STD, DST),
+            ZoneTransition(datetime(2026, 6, 1), DST, EXT),
+            ZoneTransition(datetime(2026, 9, 1), EXT, DST),
+        ], after="")
+
+        zi = self.klass.from_file(zf)
+        self.assertIsNotNone(zi)
+
     def test_zone_very_large_timestamp(self):
         """Test when a transition is in the far past or future.
 
@@ -1577,6 +1609,59 @@ class EvilZoneInfo(self.klass):
 class CZoneInfoCacheTest(ZoneInfoCacheTest):
     module = c_zoneinfo
 
+    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: RuntimeError not raised
+    def test_inconsistent_weak_cache_get(self):
+        class Cache:
+            def get(self, key, default=None):
+                return 1337
+
+        class ZI(self.klass):
+            pass
+        # Class attribute must be set after class creation
+        # to override zoneinfo.ZoneInfo.__init_subclass__.
+        ZI._weak_cache = Cache()
+
+        with self.assertRaises(RuntimeError) as te:
+            ZI("America/Los_Angeles")
+        self.assertEqual(
+            str(te.exception),
+            "Unexpected instance of int in ZI weak cache for key 'America/Los_Angeles'"
+        )
+
+    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: AttributeError not raised
+    def test_deleted_weak_cache(self):
+        class ZI(self.klass):
+            pass
+        delattr(ZI, '_weak_cache')
+
+        # These should not segfault
+        with self.assertRaises(AttributeError):
+            ZI("UTC")
+
+        with self.assertRaises(AttributeError):
+            ZI.clear_cache()
+
+    @unittest.expectedFailure  # TODO: RUSTPYTHON; AttributeError: 'int' object has no attribute '_from_cache'
+    def test_inconsistent_weak_cache_setdefault(self):
+        class Cache:
+            def get(self, key, default=None):
+                return default
+            def setdefault(self, key, value):
+                return 1337
+
+        class ZI(self.klass):
+            pass
+        # Class attribute must be set after class creation
+        # to override zoneinfo.ZoneInfo.__init_subclass__.
+        ZI._weak_cache = Cache()
+
+        with self.assertRaises(RuntimeError) as te:
+            ZI("America/Los_Angeles")
+        self.assertEqual(
+            str(te.exception),
+            "Unexpected instance of int in ZI weak cache for key 'America/Los_Angeles'"
+        )
+
 
 class ZoneInfoPickleTest(TzPathUserMixin, ZoneInfoTestBase):
     module = py_zoneinfo
diff --git a/Lib/zoneinfo/_common.py b/Lib/zoneinfo/_common.py
index 59f3f0ce853..98668c15d8b 100644
--- a/Lib/zoneinfo/_common.py
+++ b/Lib/zoneinfo/_common.py
@@ -67,6 +67,10 @@ def load_data(fobj):
             f">{timecnt}{time_type}", fobj.read(timecnt * time_size)
         )
         trans_idx = struct.unpack(f">{timecnt}B", fobj.read(timecnt))
+
+        if max(trans_idx) >= typecnt:
+            raise ValueError("Invalid transition index found while reading TZif: "
+                            f"{max(trans_idx)}")
     else:
         trans_list_utc = ()
         trans_idx = ()
diff --git a/Lib/zoneinfo/_zoneinfo.py b/Lib/zoneinfo/_zoneinfo.py
index 3ffdb4c8371..7063eb6a902 100644
--- a/Lib/zoneinfo/_zoneinfo.py
+++ b/Lib/zoneinfo/_zoneinfo.py
@@ -47,7 +47,11 @@ def __new__(cls, key):
         cls._strong_cache[key] = cls._strong_cache.pop(key, instance)
 
         if len(cls._strong_cache) > cls._strong_cache_size:
-            cls._strong_cache.popitem(last=False)
+            try:
+                cls._strong_cache.popitem(last=False)
+            except KeyError:
+                # another thread may have already emptied the cache
+                pass
 
         return instance
 
@@ -334,7 +338,7 @@ def _utcoff_to_dstoff(trans_idx, utcoffsets, isdsts):
             if not isdsts[comp_idx]:
                 dstoff = utcoff - utcoffsets[comp_idx]
 
-            if not dstoff and idx < (typecnt - 1):
+            if not dstoff and idx < (typecnt - 1) and i + 1 < len(trans_idx):
                 comp_idx = trans_idx[i + 1]
 
                 # If the following transition is also DST and we couldn't