diff --git a/build-scripts/prebuild b/build-scripts/prebuild
index 1ba3e3c..e6ee197 100755
--- a/build-scripts/prebuild
+++ b/build-scripts/prebuild
@@ -1,4 +1,21 @@
 #!/bin/bash
+set -e
 cd $(dirname $0)/..
+if ! ./generate_auto_job --check; then
+   cat 1>&2 <<EOF
+
+******************************
+
+An input or output file of generate_auto_job was modified without
+rerunning ./generate_auto_job. If you are seeing this in your pull
+request, you should pass --enable-maintainer-mode to ./configure if
+possible, or if not, run "./generate_auto_job --generate" and include
+any changed files in your commit.
+
+******************************
+
+EOF
+   exit 2
+fi
 build-scripts/build-doc
 build-scripts/download-external-libs
diff --git a/generate_auto_job b/generate_auto_job
index 64ad736..fe4e939 100755
--- a/generate_auto_job
+++ b/generate_auto_job
@@ -247,6 +247,21 @@ class Main:
                     if m:
                         old_hashes[m.group(1)] = m.group(2)
             match = old_hashes == hashes
+            if not match:
+                # Write to stdout, not stderr. What we write to stderr
+                # is visible in a normal build. Writing to stdout will
+                # hide it in that case but expose it if you directly
+                # run ./generate_auto_job --check as in CI.
+                print(f'*** {whoami} hash mismatches ***')
+                match = False
+                for k, v in hashes.items():
+                    if k not in old_hashes:
+                        print(f'  {k} is not in job.sums')
+                    elif v != old_hashes[k]:
+                        print(f'  {k} was modified')
+                for k in old_hashes:
+                    if k not in hashes:
+                        print(f'  {k} disappeared')
         except Exception:
             pass
         return match
diff --git a/job.sums b/job.sums
index 85de092..c2cc332 100644
--- a/job.sums
+++ b/job.sums
@@ -1,5 +1,5 @@
 # Generated by generate_auto_job
-generate_auto_job 751021df2a7f03c5d9a6a0ae07277004a89b9b20fa951fa4097212c72fc982a9
+generate_auto_job 464f660527880d5213ada3f5be5bdab05a425610044dbe0a2a918bc1e268726a
 include/qpdf/auto_job_c_att.hh 1f5d589ccf6684b09a203da261c31fc90a866e6eefe906b673e2d6d5a08ef753
 include/qpdf/auto_job_c_copy_att.hh 65bdce5382a60f1815214297cf005072f17556ec4a15317b600f9ecc9cb89ea4
 include/qpdf/auto_job_c_enc.hh 37ca99a124a34bb613f154c6c8934761bd7e7cc5a2719798ccec7edf05b4aeb9