First year on the job. Was already good at writing software, but bad at practices and administration. One such software was being tested live, while still in development. I was developing on the production database... .
Yeah.
I was working on an edit feature of sales records, in a table that already contained hundreds of subsidized sales of very expensive products. Based on that, the supplier had to compensate the shops with half the price of every item.
I forgot to add a where clause to the update. Lost all sales data. On production.
Asked the admin if there are backups and he says yes, checks to discover that the backup script failed for the last week (since it became live)

Whole thing was incredibly stupid. I made a ton of stupid mistakes, and so did the other people involved. The loss was around 1 year of my income. Luckily the client decided to brush it off as losses and claim some tax benefits and it all ended well.