I once checked in a database migration that was run automatically as part of the continuous deployment process we have for a client. It ran through testing and

I once checked in a database migration that was run automatically as part of the continuous deployment process we have for a client. It ran through testing and UAT without issue. When the deployment ran for production everything died. The database change was the addition of a new column to a database table which was set as NOT NULL with a default value. The production table had over 25 million rows. I learnt a valuable lesson the hard way. Never add columns large table with NOT NULL and a default value with MsSQL server. Leave it nullable.