Three months into a new job, as a senior developer (12+ years experience) and updated an import application.

With one small update query that didn't account for a possible NULL value for a parameter, so it updated all 65 million records instead of the 15 that belonged to that user.

Took 3 people and 4 days to put all the data back to it's original state.

Went right back to using the old version of the apllication, still running 2 years later. It's spaghetti code from hell with sql jobs and multiple stored procedures creating dynamic SQL, but I'm never touching it again.