Several years ago, I went to a networking event, paid my $20. Got a glass of wine. Walked into a room with 200+ people in business attire. Looked around and convinced myself that there was nobody in the room worth talking to. So, I finished my wine and left without ever talking to a soul. I only did it once and I'll never do it again!

I once did an hour-long demo that I thought covered the product really well, at just the right pace. Afterwards my colleague pointed out that my prospect wasn't really asking any questions and may not have been bought in. He then pointed out how he typically demos only a fraction of the product, and only the parts he knows ahead of time solve the prospect's problem, and after each section he pauses to ask how they address the issue today without the product. Far more compelling way to demo.

as a newer rep I had been told to look around a prospects office and ask about something to build rapport - so I asked about a family photo and the DM explained that was his family but 3 of the 4 had recently died in a horrible auto accident. I didn't get the deal!

Not hiring a professional sales and marketing consultant. 
I use the word "and" because you would be hard pressed to find a person that is a professional in both fields.  
Attempting "do it yourself" home remodeling has taught me that I should just outsource what I don't enjoy or I am not good at and do what I am good at. Now when the wife brings up a remodel project I say "absolutely, let's do it!" Then I hand her a blank check and walk away.

I was working with a prospect to close an employee benefits brokerage deal. She came to my seminar and we spent a good deal of time together. She kindly brought me in to meet her boss, the Vice President of Human Resources. I intently focused on the VP and didn't pay much attention to the person that was kind enough to invite me in. I didn't get the deal. When I was doing my post-mortem with my prospect, she told me, "you didn't look in my eyes the entire hour your were here." Last time I made that mistake.