I once tried to remove 35K contacts from a list and deleted them from the entire database instead. Tsk tsk tsk.
I would talk from a different perspective though since I am not a marketer on the client side -- I would say focusing only on the basic functionalities of email marketing and sending emails, though Eloqua is pretty bigger than that, from lead scoring, lead management, analytics, reporting, SMS integrations ... etc. and much more to be done.
We mainly need to look to it as a marketing automation platform that can empower the business functions and remove silos between Marketing & Sales.
I did the same thing. Disaster! I wasn't aware the sandbox area was available after someone in my organization told me a sandbox wasn't available.
Not putting the final destination field on the contact washing machine and overwriting the first default field instead of the desired field.
When using signatures, double check to make sure you are sending from the correct "Steve"!
Extending this a bit further I have unfortunately seen two cases where they have used terrible dummy data and placeholders including:
Dear D**khead as a placeholder for email personalization
And even worse, instead of a landing page having a dummy title of "Thank You Page" it was called the "K**k Off Page"
Not very clever!
Running a test on a campaign with merge fields on the emails, with a segment that doesn't have them.