I've put diesel in my unleaded car (my previous one, not my lovely current one [103500 miles and just had it's first repair outside of routine servicing!!] ). Fortunately, I noticed as I read the receipt on the way out of the garage, so didn't start it. I called the RAC who rescued my car and took it to my local garage. It didn't cost toooo much to drain and clean the tank before I could refill with unleaded. Doh!