diff --git a/packages/spacebars-compiler/spacebars_tests.js b/packages/spacebars-compiler/spacebars_tests.js
index 530a47bc3..01ea0f725 100644
--- a/packages/spacebars-compiler/spacebars_tests.js
+++ b/packages/spacebars-compiler/spacebars_tests.js
@@ -246,12 +246,29 @@ Tinytest.add("spacebars-compiler - parse", function (test) {
   test.equal(BlazeTools.toJS(SpacebarsCompiler.parse('{{#foo}}x{{else bar}}{{#baz}}z{{/baz}}{{/foo}}')),
              'SpacebarsCompiler.TemplateTag({type: "BLOCKOPEN", path: ["foo"], content: "x", elseContent: SpacebarsCompiler.TemplateTag({type: "BLOCKOPEN", path: ["bar"], content: SpacebarsCompiler.TemplateTag({type: "BLOCKOPEN", path: ["baz"], content: "z"})})})');
 
+  // Triple-stache in start tag — should mention raw HTML
   test.throws(function () {
     SpacebarsCompiler.parse('<a {{{x}}}></a>');
-  });
+  }, 'not allowed in an HTML start tag');
+
+  // Block helper in start tag — should suggest attribute="{{helper}}" pattern
   test.throws(function () {
     SpacebarsCompiler.parse('<a {{#if x}}{{/if}}></a>');
-  });
+  }, 'not allowed in an HTML start tag');
+
+  // Common real-world patterns that trigger this error
+  test.throws(function () {
+    SpacebarsCompiler.parse('<input {{#if isDisabled}}disabled{{/if}}>');
+  }, 'Bad syntax');
+  test.throws(function () {
+    SpacebarsCompiler.parse('<option {{#unless isActive}}selected{{/unless}}>');
+  }, 'not allowed in an HTML start tag');
+
+  // Custom block helper in start tag
+  test.throws(function () {
+    SpacebarsCompiler.parse('<a {{#myHelper}}class="foo"{{/myHelper}}></a>');
+  }, 'not allowed in an HTML start tag');
+
   test.throws(function () {
     SpacebarsCompiler.parse('<a {{k}}={[v}}></a>');
   });
@@ -261,9 +278,11 @@ Tinytest.add("spacebars-compiler - parse", function (test) {
   test.throws(function () {
     SpacebarsCompiler.parse('<a x{{y}}=z></a>');
   });
+
+  // Template inclusion in start tag — should mention it's not allowed
   test.throws(function () {
     SpacebarsCompiler.parse('<a {{> x}}></a>');
-  });
+  }, 'template inclusions are not allowed');
 
   test.equal(BlazeTools.toJS(SpacebarsCompiler.parse('<a {{! x--}} b=c{{! x}} {{! x}}></a>')),
              'HTML.A({b: "c"})');
diff --git a/packages/spacebars-compiler/templatetag.js b/packages/spacebars-compiler/templatetag.js
index bb2554532..982226de9 100644
--- a/packages/spacebars-compiler/templatetag.js
+++ b/packages/spacebars-compiler/templatetag.js
@@ -517,8 +517,35 @@ const validateTag = function (ttag, scanner) {
       scanner.fatal(`${ttag.type} template tag is not allowed in an HTML attribute`);
     }
   } else if (position === TEMPLATE_TAG_POSITION.IN_START_TAG) {
-    if (! (ttag.type === 'DOUBLE')) {
-      scanner.fatal(`Reactive HTML attributes must either have a constant name or consist of a single {{helper}} providing a dictionary of names and values.  A template tag of type ${ttag.type} is not allowed here.`);
+    if (ttag.type === 'BLOCKOPEN') {
+      const path0 = ttag.path[0];
+      const isBuiltIn = ttag.path.length === 1 && (path0 === 'if' || path0 === 'unless' || path0 === 'with' || path0 === 'each');
+      if (isBuiltIn) {
+        scanner.fatal(
+          `{{#${path0}}} is not allowed in an HTML start tag. ` +
+          'To conditionally add a boolean attribute like "disabled", "checked", or "selected", ' +
+          'use the attribute="{{helper}}" syntax instead.\n' +
+          '  Bad syntax:  <input {{#if isDisabled}}disabled{{/if}}>\n' +
+          '  Good syntax: <input disabled="{{isDisabled}}">'
+        );
+      } else {
+        scanner.fatal(
+          `{{#${path0}}} is not allowed in an HTML start tag. ` +
+          'Only a single {{helper}} returning a dictionary of attribute name=value pairs is allowed here.'
+        );
+      }
+    } else if (ttag.type === 'TRIPLE') {
+      scanner.fatal(
+        'Triple-stache {{{...}}} is not allowed in an HTML start tag because it outputs raw HTML. ' +
+        'Use a double-stache {{helper}} that returns a dictionary of attribute name=value pairs instead.'
+      );
+    } else if (ttag.type === 'INCLUSION') {
+      scanner.fatal(
+        `{{> ...}} template inclusions are not allowed in an HTML start tag. ` +
+        'Only a single {{helper}} returning a dictionary of attribute name=value pairs is allowed here.'
+      );
+    } else if (ttag.type !== 'DOUBLE') {
+      scanner.fatal(`A template tag of type "${ttag.type}" is not allowed in an HTML start tag.`);
     }
     if (scanner.peek() === '=') {
       scanner.fatal("Template tags are not allowed in attribute names, only in attribute values or in the form of a single {{helper}} that evaluates to a dictionary of name=value pairs.");