diff --git a/mlir/docs/DialectConversion.md b/mlir/docs/DialectConversion.md
--- a/mlir/docs/DialectConversion.md
+++ b/mlir/docs/DialectConversion.md
@@ -75,6 +75,10 @@
         conversion to be successful. This action also allows for selectively
         marking specific operations as illegal in an otherwise legal dialect.
 
+Operations and dialects that are neither explicitly marked legal nor illegal are
+separate from the above ("unknown" operations) and are treated differently for
+example for the purposes of partial conversion as mentioned above.
+
 An example conversion target is shown below:
 
 ```c++
diff --git a/mlir/lib/Transforms/Utils/DialectConversion.cpp b/mlir/lib/Transforms/Utils/DialectConversion.cpp
--- a/mlir/lib/Transforms/Utils/DialectConversion.cpp
+++ b/mlir/lib/Transforms/Utils/DialectConversion.cpp
@@ -1931,7 +1931,7 @@
     Operation *cstOp = rewriterImpl.createdOps[i];
     if (failed(legalize(cstOp, rewriter))) {
       LLVM_DEBUG(logFailure(rewriterImpl.logger,
-                            "generated constant '{0}' was illegal",
+                            "generated constant '{0}' failed to legalize",
                             cstOp->getName()));
       rewriterImpl.resetState(curState);
       return failure();
@@ -2106,7 +2106,7 @@
     Operation *op = impl.createdOps[i];
     if (failed(legalize(op, rewriter))) {
       LLVM_DEBUG(logFailure(impl.logger,
-                            "generated operation '{0}'({1}) was illegal",
+                            "generated operation '{0}'({1}) failed to legalize",
                             op->getName(), op));
       return failure();
     }
@@ -2120,9 +2120,9 @@
   for (int i = state.numRootUpdates, e = newState.numRootUpdates; i != e; ++i) {
     Operation *op = impl.rootUpdates[i].getOperation();
     if (failed(legalize(op, rewriter))) {
-      LLVM_DEBUG(logFailure(impl.logger,
-                            "operation updated in-place '{0}' was illegal",
-                            op->getName()));
+      LLVM_DEBUG(logFailure(
+          impl.logger, "operation updated in-place '{0}' failed to legalize",
+          op->getName()));
       return failure();
     }
   }