--- a/tests.c	Sat Dec 28 09:54:57 2019 +0200
+++ b/tests.c	Sat Dec 28 09:55:37 2019 +0200
@@ -25,7 +25,7 @@
 typedef struct
 {
     char *header;
-    BOOL shown;
+    BOOL shown, failed;
 } test_ctx;
 
 
@@ -183,6 +183,8 @@
             tprint(0, "\n");
         }
         tests_failed++;
+
+        ctx->failed = TRUE;
     }
 }
 
@@ -210,6 +212,10 @@
     va_list tmp;
     test_ctx ctx;
 
+    // Setup printf debug value
+    th_printf_debug = th_verbosity >= 3;
+    th_printf_debug_prefix = "  - ";
+
     // Test basic *printf() functionality
     test_start(&ctx, "th_vsnprintf(%" PRIu_SIZE_T ", \"%s\", %s)", len, fmt, msg);
 
@@ -228,6 +234,13 @@
     test_result_msg(&ctx, (unsigned char) buf2[len] == SET_SENTINEL_BYTE, "buffer #2 overflow, sentinel 0x%02x", buf2[len]);
     }
 
+    if (ctx.failed && !th_printf_debug && th_verbosity >= 1)
+    {
+        th_printf_debug = TRUE;
+        va_copy(tmp, ap); ret1 = th_vsnprintf(buf1, len, fmt, tmp);
+        va_copy(tmp, ap); ret2 = vsnprintf(buf2, len, fmt, tmp);
+    }
+
     test_end(&ctx);
 }
 
@@ -563,8 +576,6 @@
 
     tprint(1, "Enabled test types are 0x%04x.\n", optFlags);
 
-    // Setup printf debug value
-    th_printf_debug = th_verbosity >= 3;
 
     //
     // Test series for printf()