What was your the biggest marketing mistake?
One early mistake I made was to vastly over-estimate the interest my (assumed) target audience would have in a particular range of services. In a classic case of "plumber with a leaky tap", I sent out '000s of postcards offering a service no-one was interested in. Result: 3 (just three!) enquiries.
I didn't research the market and ask them : a. do you need this service; b. can you afford this service; c. are you the right audience for this service? It taught me an expensive lesson – it's not about what you DO, it's about what they GET.
The biggest mistake I made was too focus too much on SEO and too little on the actual consumer.
My sites all rank without a problem, but I found adding a human element to my content made a big difference in conversions.