When a move between servers went wrong.
THE SITUATION
Files or data did not transfer completely, or the new environment behaves differently, and the site is broken.
FIRST
Do not cancel the old environment. It is your fallback, and it should still be running.
If DNS has already switched, you can point it back while you resolve the problem.
WHAT USUALLY GOES WRONG
Database credentials not updated for the new prefix File permissions or ownership wrong after transfer A different PHP version A missing PHP extension Absolute paths in configuration pointing at the old server Email not recreated Cron jobs not recreated
DIAGNOSING
Read the error log first. It usually names the problem directly.
Compare the PHP version and extensions against the old environment.
RESTORING FROM THE MIGRATION BACKUP
If the new environment is unusable, restore your pre-migration backup to the old server, or start the migration again.
THE LESSON
Test with a hosts file entry before changing DNS. Nearly every migration disaster is caused by switching before verifying.
