One summer in college, I got an internship at a company that made health information systems. After fixing bugs in PHP scripts for a couple weeks, I was granted access to their production DB. (Hey, they were short on talent.) This database stored all kinds of stuff, including the operating room schedules for various hospitals. It included who was being operated on, when, what operation they were scheduled for, and important information such as patient allergies, malignant hyperthermia, etc.

I was a little sleepy one morning and accidentally connected to prod instead of testing. I thought, "That's weird, this UPDATE shouldn't have taken so long-oh shit." I'd managed to clear all allergy and malignant hyperthermia fields. For all I knew, some anesthesiologist would kill a patient because of my mistake. I was shaking. I immediately found the technical lead, pulled him from a meeting, and told him what happened. He'd been smart enough to set up hourly DB snapshots and query logs. It only took five minutes to restore from a snapshot and replay all the logs, not including my UPDATE.

Afterwards, my access to prod was not revoked. We both agreed I'd learned a valuable lesson, and that I was unlikely to repeat that mistake. The tech lead explained the incident to the higher-ups, who decided to avoid mentioning anything to the affected hospitals.

If it's any consolation, the company is no longer in business.

Just remember when you screw things up: Your mistake probably won't get anyone killed, so don't panic too much.