What was the biggest production incident that you caused?

My first job out of college I accidentally sent a test email to everyone whose email address was in our database (company practice was to use copies of the live db to test on).

Shortly after, I wrote a script for myself that would switch all the email addresses in the database to my email address after copying the DB.

Downside: I didn't have a company email address, so I used my personal address, and either they found that script and are still using it, or they're still using old copies of the test DBs, because it's been nearly a decade and I'm still getting test emails from that system.

@HollowKitty perhaps an example of "if it's not broke, don't fix it." - for decades apparently.

Took down a core switch, (and thereby the most of the corporate network) during a busy and hectic afternoon because I accidentally set a trunk link to an access link. Woopsies.