diff --git a/dist/wcpatch/src/main.cpp b/dist/wcpatch/src/main.cpp
index 09d96ae..4785572 100644
--- a/dist/wcpatch/src/main.cpp
+++ b/dist/wcpatch/src/main.cpp
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <iostream>
+#include <iomanip>
 #include <map>
 #include <optional>
 #include <string>
@@ -401,7 +402,16 @@ bool apply_dif(stdext::multi_ref<stdext::stream, stdext::seekable> file_data, ui
         seekable.seek(stdext::seek_from::begin, offset);
         auto value = stream.read<std::byte>();
         if (value != original_value)
+        {
+            unsigned file_b = std::to_integer<unsigned>(value);
+            unsigned expect_b = std::to_integer<unsigned>(original_value);
+            unsigned repl_b = std::to_integer<unsigned>(replacement_value);
+            std::cerr << "apply_dif: mismatch at offset 0x" << std::hex << offset
+                      << ": file=0x" << std::setw(2) << std::setfill('0') << file_b
+                      << " expected=0x" << std::setw(2) << expect_b
+                      << " replace=0x" << std::setw(2) << repl_b << std::dec << "\n";
             return false;
+        }
         seekable.seek(stdext::seek_from::current, -1);
         stream.write(replacement_value);
     }