I did something like that once, I wasn't very good at SQL but I needed some data, so I logged in into the production database and run my SELECT queries, I didn't change anything so everything was good, or so I thought.
I created a cross product over tables with millions of entries and when it didn't respond I thought it was odd but it was time to go home anyway. On the way home they called me and asked what I did. They had to restart the DB server because once the cache timed out one application after another started failing.
Good companies wouldn't fire someone for this because:
There should be processes in place to prevent this, or recover from this, anyway. It's a team/department failure and you would just be the straw that broke the camel's back.
They now know you've experienced this and will hopefully know to never do it again. Bringing in someone else could just reintroduce the issue.