Fixed #30261 -- Prevented Form._html_output() from mutating errors if hidden fields have errors.

2 files changed, 7 insertions, 1 deletions

diff --git a/django/forms/forms.py b/django/forms/forms.py
index a601467e18..81c59dea33 100644
--- a/django/forms/forms.py
+++ b/django/forms/forms.py
@@ -191,7 +191,8 @@ class BaseForm:
 
     def _html_output(self, normal_row, error_row, row_ender, help_text_html, errors_on_separate_row):
         "Output HTML. Used by as_table(), as_ul(), as_p()."
-        top_errors = self.non_field_errors()  # Errors that should be displayed above all fields.
+        # Errors that should be displayed above all fields.
+        top_errors = self.non_field_errors().copy()
         output, hidden_fields = [], []
 
         for name, field in self.fields.items():
diff --git a/django/forms/utils.py b/django/forms/utils.py
index 5ae137943a..fbe79f1142 100644
--- a/django/forms/utils.py
+++ b/django/forms/utils.py
@@ -92,6 +92,11 @@ class ErrorList(UserList, list):
     def as_data(self):
         return ValidationError(self.data).error_list
 
+    def copy(self):
+        copy = super().copy()
+        copy.error_class = self.error_class
+        return copy
+
     def get_json_data(self, escape_html=False):
         errors = []
         for error in self.as_data():