diff --git a/mercurial/branching/rev_cache.py b/mercurial/branching/rev_cache.py
--- a/mercurial/branching/rev_cache.py
+++ b/mercurial/branching/rev_cache.py
@@ -56,23 +56,28 @@ class rbcrevs:
         self._prefix = revs
         self._rest = bytearray()
 
+    @property
+    def len_prefix(self):
+        size = len(self._prefix)
+        return size - (size % _rbcrecsize)
+
     def __len__(self):
-        return len(self._prefix) + len(self._rest)
+        return self.len_prefix + len(self._rest)
 
     def unpack_record(self, rbcrevidx):
-        if rbcrevidx < len(self._prefix):
+        if rbcrevidx < self.len_prefix:
             return unpack_from(_rbcrecfmt, util.buffer(self._prefix), rbcrevidx)
         else:
             return unpack_from(
                 _rbcrecfmt,
                 util.buffer(self._rest),
-                rbcrevidx - len(self._prefix),
+                rbcrevidx - self.len_prefix,
             )
 
     def make_mutable(self):
-        if len(self._prefix) > 0:
+        if self.len_prefix > 0:
             entirety = bytearray()
-            entirety[:] = self._prefix
+            entirety[:] = self._prefix[: self.len_prefix]
             entirety.extend(self._rest)
             self._rest = entirety
             self._prefix = bytearray()
@@ -82,10 +87,10 @@ class rbcrevs:
         del self._rest[pos:]
 
     def pack_into(self, rbcrevidx, node, branchidx):
-        if rbcrevidx < len(self._prefix):
+        if rbcrevidx < self.len_prefix:
             self.make_mutable()
         buf = self._rest
-        start_offset = rbcrevidx - len(self._prefix)
+        start_offset = rbcrevidx - self.len_prefix
         end_offset = start_offset + _rbcrecsize
 
         if len(self._rest) < end_offset:
@@ -107,14 +112,14 @@ class rbcrevs:
         return self._rest.extend(extension)
 
     def slice(self, begin, end):
-        if begin < len(self._prefix):
+        if begin < self.len_prefix:
             acc = bytearray()
-            acc[:] = self._prefix[begin:end]
+            acc[:] = self._prefix[begin : min(end, self.len_prefix)]
             acc.extend(
-                self._rest[begin - len(self._prefix) : end - len(self._prefix)]
+                self._rest[begin - self.len_prefix : end - self.len_prefix]
             )
             return acc
-        return self._rest[begin - len(self._prefix) : end - len(self._prefix)]
+        return self._rest[begin - self.len_prefix : end - self.len_prefix]
 
 
 class revbranchcache:
@@ -388,6 +393,9 @@ class revbranchcache:
         if self._force_overwrite:
             start = 0
 
+        # align start on entry boundary
+        start = _rbcrecsize * (start // _rbcrecsize)
+
         with repo.cachevfs.open(_rbcrevs, b'a+b') as f:
             pass  # this make sure the file exist…
         with repo.cachevfs.open(_rbcrevs, b'r+b') as f:
diff --git a/tests/test-branches.t b/tests/test-branches.t
--- a/tests/test-branches.t
+++ b/tests/test-branches.t
@@ -831,15 +831,30 @@ no errors when wlock cannot be acquired
   $ mv .hg/cache/rbc-revs-v2_ .hg/cache/rbc-revs-v2
 #endif
 
-recovery from invalid cache revs file with trailing data
-  $ echo >> .hg/cache/rbc-revs-v2
+dealing with valid cache revs file but for extra trailing data
+--------------------------------------------------------------
+
+When the trailing data are smaller than a record, they are practically
+invisible to the cache and ignored. No warning is issued about them.
+
+  $ echo '42' >> .hg/cache/rbc-revs-v2
   $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
   5
-  cache/rbc-revs-v2 contains 2 unknown trailing bytes
   $ f --size .hg/cache/rbc-revs*
-  .hg/cache/rbc-revs-v2: size=162
+  .hg/cache/rbc-revs-v2: size=164
+
+When the trailing data are larger than a record, they are seens as extra
+(probably invalid) data. We warn about them when writing.
+
+  $ echo 'abracadabra!' >> .hg/cache/rbc-revs-v2
+  $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
+  5
+  cache/rbc-revs-v2 contains 17 unknown trailing bytes
+  $ f --size .hg/cache/rbc-revs*
+  .hg/cache/rbc-revs-v2: size=177
 
 recovery from invalid cache file with partial last record
+---------------------------------------------------------
   $ mv .hg/cache/rbc-revs-v2 .
   $ f -qDB 119 rbc-revs-v2 > .hg/cache/rbc-revs-v2
   $ f --size .hg/cache/rbc-revs*