I have witnessed the following test failure several times on the CI. Restart always solved it.
https://git.drupalcode.org/issue/drupal-3278759/-/pipelines/336376/test_...
Drupal\Tests\layout_builder\Functional\LayoutBuilderBlocksTest::testBlockPlaceholder
Behat\Mink\Exception\ResponseTextException: The text "Placeholder for the "The block label" block" appears in the text of this page, but it should not.
vendor/behat/mink/src/WebAssert.php:907
vendor/behat/mink/src/WebAssert.php:312
core/modules/layout_builder/tests/src/Functional/LayoutBuilderBlocksTest.php:219
Active
11.0 🔥
layout_builder.module
Closing a dupe of 🐛 [random test failure] LayoutBuilderBlocksTest::testBlockPlaceholder failing Active
This is being tracked in the meta also: 🌱 [meta] Known intermittent, random, and environment-specific test failures Active
I just ran into this in an issue I was working on.