diff --git a/mercurial/vfs.py b/mercurial/vfs.py
--- a/mercurial/vfs.py
+++ b/mercurial/vfs.py
@@ -303,6 +303,53 @@
             finally:
                 vfs._backgroundfilecloser = None
 
+class _appendfileproxy(object):
+    """Proxy type to prevent .tell() before .seek() or .write() append files.
+
+    Files opened for append aren't required to have a meaningful fpos
+    after open, so we require a write or explicit seek before allowing
+    .tell() on those files.
+    """
+    def __init__(self, fp):
+        object.__setattr__(self, r'_fp', fp)
+        object.__setattr__(self, r'_tellok', False)
+
+    def tell(self):
+        if not self._tellok:
+            raise error.ProgrammingError(
+                'tell() on append-mode file requires write() or seek() first')
+        return self._fp.tell()
+
+    def seek(self, *args, **kwargs):
+        object.__setattr__(self, r'_tellok', True)
+        return self._fp.seek(*args, **kwargs)
+
+    def write(self, *args, **kwargs):
+        object.__setattr__(self, r'_tellok', True)
+        return self._fp.write(*args, **kwargs)
+
+    def __repr__(self):
+        return r'<_appendfileproxy of %r>' % self._fp
+
+    # __magic__ methods get looked up on the type, not the instance,
+    # so we have to proxy __enter__ and __exit__ by hand.
+    def __enter__(self):
+        self._fp.__enter__()
+        return self
+
+    def __exit__(self, *args, **kwargs):
+        self._fp.__exit__(*args, **kwargs)
+
+    # proxy all other methods
+    def __getattr__(self, attr):
+        return getattr(self._fp, attr)
+
+    def __setattr__(self, attr, value):
+        return setattr(self._fp, attr, value)
+
+    def __delattr__(self, attr):
+        return delattr(self._fp, attr)
+
 class vfs(abstractvfs):
     '''Operate files relative to a base directory
 
@@ -441,7 +488,8 @@
                                   )
 
             fp = delayclosedfile(fp, self._backgroundfilecloser)
-
+        if mode in ('a', 'ab'):
+            return _appendfileproxy(fp)
         return fp
 
     def symlink(self, src, dst):