Make sure that this is the ONLY code in your window or that you are protected by a RETURN or SET EXECUTION OFF at the top of your screen. I have this put in place by default on new query windows. This protects you from running too much code by accident.
Make a habit of checking what instance you are connected to before running any ad-hoc code. Running code meant for a model or test environment in production can be a very scary thing.
This is good advice.