Tell us what was your biggest marketing/advertising mistake? Where and what have you spent money on that left you extremely disappointed with results?

Best,

When I first started using AdWords I set very high daily limits (I know a newbie mistake.) Well I got busy and forgot to check my adwords account for several days. When I finally looked I saw that I hade spent several hundred dollars in ads, yet I didn't get any conversions

It was an expensive lesson to learn

Ouch, indeed it was.
I had a similar experience. I purchased a very expensive text-ad on a well-known resource through AdBrite, and, well, forgot to turn it off after the first time it ran... It ran 2 more times until I noticed my VISA statement. That mistake cost me $800 in extra charges.

I once blew $800 on a one month banner buy at a Howard Stern forum 6 years ago. It's all about TARGETTING.... that's the lesson I learned. VOLUME is worthless if it doesn't convert.

I believe my biggest mistake was web hosting chat and hostingcatalog.com when I first started out. I ran through a couple hundred dollars in three days and decided that was done. I am awaiting to see how Google Adwords works out along with some other ideas upcomming.