Why features pass QA and still break in production
Database migrations are where the mock data problem shows up most clearly. A migration that adds an index to a table with 500 rows in the development database runs in milliseconds and passes every test. The same migration against a production table with 8 million rows locks the table for 90 seconds during peak traffic. Nobody saw it coming because nobody tested it against 8 million rows. This isn't an edge case.
