Index: llvm/include/llvm/Support/FileCheck.h
===================================================================
--- llvm/include/llvm/Support/FileCheck.h
+++ llvm/include/llvm/Support/FileCheck.h
@@ -164,6 +164,7 @@
   /// example, there might be a fuzzy match after a fail.
   enum MatchType {
     // TODO: More members will appear with later patches in this series.
+    MatchFinalButIllegal,  //< the final but illegal match for an expected pattern
     MatchNoneButExpected,  //< no match for an expected pattern
     MatchFuzzy,            //< a fuzzy match (because no perfect match)
     MatchTypeCount,
Index: llvm/lib/Support/FileCheck.cpp
===================================================================
--- llvm/lib/Support/FileCheck.cpp
+++ llvm/lib/Support/FileCheck.cpp
@@ -1047,17 +1047,27 @@
   // Similar to the above, in "label-scan mode" we can't yet handle CHECK-NEXT
   // or CHECK-NOT
   if (!IsLabelScanMode) {
-    StringRef SkippedRegion = Buffer.substr(LastPos, FirstMatchPos - LastPos);
+    size_t MatchPos = FirstMatchPos - LastPos;
+    StringRef MatchBuffer = Buffer.substr(LastPos);
+    StringRef SkippedRegion = Buffer.substr(LastPos, MatchPos);
 
     // If this check is a "CHECK-NEXT", verify that the previous match was on
     // the previous line (i.e. that there is one newline between them).
-    if (CheckNext(SM, SkippedRegion))
+    if (CheckNext(SM, SkippedRegion)) {
+      ProcessMatchResult(FileCheckDiag::MatchFinalButIllegal, SM, Loc,
+                         Pat.getCheckTy(), MatchBuffer, MatchPos, MatchLen,
+                         Diags);
       return StringRef::npos;
+    }
 
     // If this check is a "CHECK-SAME", verify that the previous match was on
     // the same line (i.e. that there is no newline between them).
-    if (CheckSame(SM, SkippedRegion))
+    if (CheckSame(SM, SkippedRegion)) {
+      ProcessMatchResult(FileCheckDiag::MatchFinalButIllegal, SM, Loc,
+                         Pat.getCheckTy(), MatchBuffer, MatchPos, MatchLen,
+                         Diags);
       return StringRef::npos;
+    }
 
     // If this match had "not strings", verify that they don't exist in the
     // skipped region.
Index: llvm/test/FileCheck/dump-input-annotations.txt
===================================================================
--- llvm/test/FileCheck/dump-input-annotations.txt
+++ llvm/test/FileCheck/dump-input-annotations.txt
@@ -86,7 +86,7 @@
 ; CNT-NOT:    {{.}}
 
 ;--------------------------------------------------
-; CHECK-NEXT (also: EOF search-range)
+; CHECK-NEXT (also: EOF search-range, illegal match)
 ;--------------------------------------------------
 
 ; Good match and no match.
@@ -113,8 +113,25 @@
 ; NXT-NEXT:   >>>>>>
 ; NXT-NOT:    {{.}}
 
+; Illegal match.
+
+; RUN: echo 'yonder' >> %t.in
+; RUN: echo 'world' >> %t.in
+
+; RUN: not FileCheck -dump-input=always -input-file %t.in %t.chk 2>&1 \
+; RUN: | FileCheck -match-full-lines %s -check-prefix=NXT2
+
+; NXT2:      <<<<<<
+; NXT2-NEXT:         1: hello
+; NXT2-NEXT:         2: again
+; NXT2-NEXT:         3: yonder
+; NXT2-NEXT:         4: world
+; NXT2-NEXT: next:3     !~~~~
+; NXT2-NEXT: >>>>>>
+; NXT2-NOT:  {{.}}
+
 ;--------------------------------------------------
-; CHECK-SAME (also: single-char search range)
+; CHECK-SAME (also: single-char search range, illegal match)
 ;--------------------------------------------------
 
 ; Good match and no match.
@@ -138,8 +155,22 @@
 ; SAM-NEXT:   >>>>>>
 ; SAM-NOT:    {{.}}
 
+; Illegal match.
+
+; RUN: echo 'again' >> %t.in
+
+; RUN: not FileCheck -dump-input=always -input-file %t.in %t.chk -v 2>&1 \
+; RUN: | FileCheck -match-full-lines %s -check-prefixes=SAM2
+
+; SAM2:      <<<<<<
+; SAM2-NEXT:          1: hello world!
+; SAM2-NEXT:          2: again
+; SAM2-NEXT: same:3      !~~~~
+; SAM2-NEXT: >>>>>>
+; SAM2-NOT:  {{.}}
+
 ;--------------------------------------------------
-; CHECK-EMPTY (also: search range ends at label)
+; CHECK-EMPTY (also: search range ends at label, illegal match)
 ;--------------------------------------------------
 
 ; Good match and no match.
@@ -176,6 +207,29 @@
 ; EMP-NEXT:   >>>>>>
 ; EMP-NOT:    {{.}}
 
+; Illegal match.
+
+; RUN: echo 'hello' > %t.in
+; RUN: echo 'world' >> %t.in
+
+; RUN: echo 'CHECK: hello' > %t.chk
+; RUN: echo 'CHECK-EMPTY:' >> %t.chk
+
+; RUN: not FileCheck -dump-input=always -input-file %t.in %t.chk 2>&1 \
+; RUN: | FileCheck -match-full-lines %s -check-prefix=EMP2
+; RUN: not FileCheck -dump-input=always -input-file %t.in %t.chk -v 2>&1 \
+; RUN: | FileCheck -match-full-lines %s -check-prefixes=EMP2,EMP2-V
+; RUN: not FileCheck -dump-input=always -input-file %t.in %t.chk -vv 2>&1 \
+; RUN: | FileCheck -match-full-lines %s -check-prefixes=EMP2,EMP2-V,EMP2-VV
+
+; EMP2:        <<<<<<
+; EMP2-NEXT:            1: hello
+; EMP2-NEXT:            2: world
+; EMP2-NEXT:            3:
+; EMP2-NEXT:   empty:2     !
+; EMP2-NEXT:   >>>>>>
+; EMP2-NOT:    {{.}}
+
 ;--------------------------------------------------
 ; CHECK-DAG
 ;--------------------------------------------------
Index: llvm/utils/FileCheck/FileCheck.cpp
===================================================================
--- llvm/utils/FileCheck/FileCheck.cpp
+++ llvm/utils/FileCheck/FileCheck.cpp
@@ -138,6 +138,8 @@
 
 static MatchTypeStyle GetMatchTypeStyle(unsigned MatchTy) {
   switch (MatchTy) {
+  case FileCheckDiag::MatchFinalButIllegal:
+    return MatchTypeStyle('!', raw_ostream::RED);
   case FileCheckDiag::MatchNoneButExpected:
     return MatchTypeStyle('X', raw_ostream::RED);
   case FileCheckDiag::MatchFuzzy:
@@ -170,8 +172,13 @@
 
   // Markers on annotation lines.
   OS << "  - ";
+  WithColor(OS, raw_ostream::SAVEDCOLOR, true) << "!~~";
+  OS << "    marks bad match, such as:\n"
+     << "           - CHECK-NEXT on same line as previous match (error)\n"
+     << "  - ";
   WithColor(OS, raw_ostream::SAVEDCOLOR, true) << "X~~";
-  OS << "    marks search range when no match is found\n"
+  OS << "    marks search range when no match is found, such as:\n"
+     << "           - CHECK-NEXT not found (error)\n"
      << "  - ";
   WithColor(OS, raw_ostream::SAVEDCOLOR, true) << "?";
   OS << "      marks fuzzy match when no match is found\n";