diff --git a/src/calibre/gui2/dialogs/tag_categories.py b/src/calibre/gui2/dialogs/tag_categories.py
index 13172b73c7..5e27c37364 100644
--- a/src/calibre/gui2/dialogs/tag_categories.py
+++ b/src/calibre/gui2/dialogs/tag_categories.py
@@ -284,11 +284,19 @@ class TagCategories(QDialog, Ui_TagCategories):
                       'or after periods.')).exec()
             return False
         for c in sorted(self.user_categories.keys(), key=primary_sort_key):
-            if strcmp(c, cat_name) == 0 or \
-                    (icu_lower(cat_name).startswith(icu_lower(c) + '.') and
-                     not cat_name.startswith(c + '.')):
+            if strcmp(c, cat_name) == 0:
                 error_dialog(self, _('Name already used'),
-                        _('That name is already used, perhaps with different case.')).exec()
+                        _('The user category name is already used, perhaps with different case.'),
+                        det_msg=_('Existing category: {existing}\nNew category: {new}').format(existing=c, new=cat_name),
+                        show=True)
+                return False
+            if icu_lower(cat_name).startswith(icu_lower(c) + '.') and not cat_name.startswith(c + '.'):
+                error_dialog(self, _('Name already used'),
+                        _('The hierarchical prefix of the new category is already used, '
+                          'perhaps with different case.'),
+                        det_msg=_('Existing prefix: {prefix}\n'
+                                  'New category: {new}').format(prefix=c, new=cat_name),
+                        show=True)
                 return False
         if cat_name not in self.user_categories:
             self.user_categories[cat_name] = set()
@@ -316,7 +324,10 @@ class TagCategories(QDialog, Ui_TagCategories):
         for c in self.user_categories:
             if strcmp(c, cat_name) == 0:
                 error_dialog(self, _('Name already used'),
-                        _('That name is already used, perhaps with different case.')).exec()
+                        _('The user category name is already used, perhaps with different case.'),
+                        det_msg=_('Existing category: {existing}\n'
+                                  'New category: {new}').format(existing=c, new=cat_name),
+                        show=True)
                 return
         # The order below is important because of signals
         self.user_categories[cat_name] = self.user_categories[self.current_cat_name]
diff --git a/src/calibre/gui2/dialogs/tag_list_editor.py b/src/calibre/gui2/dialogs/tag_list_editor.py
index 0c78e25cab..e5d2fb5a17 100644
--- a/src/calibre/gui2/dialogs/tag_list_editor.py
+++ b/src/calibre/gui2/dialogs/tag_list_editor.py
@@ -647,16 +647,11 @@ class TagListEditor(QDialog, Ui_TagListEditor):
         self.table.blockSignals(False)
 
     def selection_changed(self):
-        col0 = tuple(item for item in self.table.selectedItems() if item.column() == 0)
-        col3 = tuple(item for item in self.table.selectedItems() if item.column() == 3)
-        if col0 and col3:
-            error_dialog(self, _('Cannot select in multiple columns'),
-                               '<p>'+_('Selection of items in multiple columns is not supported. '
-                                       'The selection will be cleared')+'<br>',
-                               show=True)
-            sm = self.table.selectionModel()
+        if self.table.currentIndex().isValid():
+            col = self.table.currentIndex().column()
             self.table.blockSignals(True)
-            sm.clear()
+            for itm in (item for item in self.table.selectedItems() if item.column() != col):
+                itm.setSelected(False)
             self.table.blockSignals(False)
 
     def check_for_deleted_items(self, show_error=False):
diff --git a/src/calibre/gui2/tag_browser/view.py b/src/calibre/gui2/tag_browser/view.py
index 7e21e755f3..dad222f99a 100644
--- a/src/calibre/gui2/tag_browser/view.py
+++ b/src/calibre/gui2/tag_browser/view.py
@@ -722,7 +722,16 @@ class TagsView(QTreeView):  # {{{
                 added_show_hidden_categories = True
                 m = self.context_menu.addMenu(_('Show category'))
                 m.setIcon(QIcon.ic('plus.png'))
-                for col in sorted(self.hidden_categories,
+                # The search category can disappear from field_metadata. Perhaps
+                # other dynamic categories can as well. The implication is that
+                # dynamic categories are being removed, but how that would
+                # happen is a mystery. I suspect a plugin is operating on the
+                # "real" field_metadata instead of a copy, thereby changing the
+                # dict used by the rest of calibre.
+                #
+                # As it can happen, to avoid key errors check that a category
+                # exists before offering to unhide it.
+                for col in sorted((c for c in self.hidden_categories if c in self.db.field_metadata),
                         key=lambda x: sort_key(self.db.field_metadata[x]['name'])):
                     ac = m.addAction(self.db.field_metadata[col]['name'],
                         partial(self.context_menu_handler, action='show', category=col))