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.