1. Most common mistake is too much text and pictures in one page. The visual becomes noisy and it is difficult to understand which information is most important. It would be better to choose one message and think about one action that your potential client have to take.
2. When the ad looks good but somehow the main message gets wrong. The message always goes first, the visual content should help to communicate and make the others understand what you want to say but it doesn't have to disturb. To make it right you need to have an answer to this question: what are we trying to accomplish with our visual content?
If you want to avoid these common mistakes, before creating an ad you need to answer these questions:
What are we trying to accomplish with our visual content?
Who is our audience and what content do they crave?
What problems does our organization solve?
How can we best position our business or brand and create a consistent look and feel?
What is our clearly defined vision of who we are and what makes us unique? How can we communicate those messages in a compelling way?
What metrics will we use to measure success? For which terms should this image appear in search engine results?