--- a/tests/test-fileset.t	Mon Dec 21 22:31:16 2015 +0900
+++ b/tests/test-fileset.t	Mon Dec 21 22:31:16 2015 +0900
@@ -328,3 +328,22 @@
   b2link
   bin
   c1
+
+Test detection of unintentional 'matchctx.existing()' invocation
+
+  $ cat > $TESTTMP/existingcaller.py <<EOF
+  > from mercurial import fileset
+  > 
+  > @fileset.predicate('existingcaller()', callexisting=False)
+  > def existingcaller(mctx, x):
+  >     # this 'mctx.existing()' invocation is unintentional
+  >     return [f for f in mctx.existing()]
+  > EOF
+
+  $ cat >> .hg/hgrc <<EOF
+  > [extensions]
+  > existingcaller = $TESTTMP/existingcaller.py
+  > EOF
+
+  $ fileset 'existingcaller()' 2>&1 | tail -1
+  AssertionError: unexpected existing() invocation