Agree with @shadowlmd and similar annoyances persist in 5.9. Some of us keep a close watch on error logs to stay ahead of potential problems.
No part of the system should be throwing undefined constant errors simply because an abusive end-user is doing something they should not. Responsible operators get stuck sifting through WP error log when trying to identify and isolate any real problems.
Use of undefined constant ABSPATH – assumed ‘ABSPATH’ as of WP5.9
