Practically every SEO has a horror story or two about certain past clients. Is there some way we can tell in advance that a job is going to be problematic? Yes, there are certain warning signs. Itai Levitan discusses some of the most common ones.
Just about every SEO that I've met has a passion for search engine optimization. It's a mysterious and intriguing practice, and we SEOs love it! It's fun to tweak those pages and receive almost immediate feedback as search engine ranking fluctuation are realized. SEO is never boring, because each website is unique, as are the various stages of the search engine optimization work process. SEOs love everything about it, from the initial investigation of the site and its market, learning about what the website is trying to achieve and how; following through to the keyword research and strategy, the optimization, link building, monitoring and improvements. Throughout the SEO process, SEOs are almost always feeling a positive tension and inner drive. They are inspired to work those late hours, knowing that SEO is probably going to pop up in their dreams anyway!
There are various reasons why almost every experienced SEO has taken at least one project that she or he later regretted. It's simply hard to turn down a SEO project, whether you are servicing a client, optimizing your own site, helping a friend out or what have you. Most SEOs like the challenge, and are not afraid to carve their way in the world. Some see it as a secret riddle that is calling out to be solved, and can't refuse the challenge. Other SEOs, who are freelancers or employees of SEO firms, simply need to optimize another website as part of their work.
But in certain cases, "just say no" applies to SEOs as well. There are many reasons why this can be true. For example, some projects are just too much trouble and are aiming too high while lacking sufficient time and/or budget to get the job done right. There are a number of situations where we SEOs should stop to look at certain warning signs before diving into another optimization project.
So what happens when you take on a SEO project that you should not have accepted? Well, first there are the disappointing results. Not only to yourself, but to other stakeholders and especially - when relevant - to clients. I am a great believer in (1) reputation, which cannot be directly purchased and needs to be earned and (2) motivation, which is essentially the core of every success story out there, including SEO success stories. Unfortunately, I have seen myself and others fall at least once with a project that should not have been agreed to in the first place. Fortunately, there was no damage to reputation, but there was some damage to my motivation, at least in the short term. Why is this important? Because that can also cause the SEO to lose his/her cool or logic, and these two traits are key to SEO success.
How can we SEOs identify the situations where we're going to be sorry later that we took on that problematic SEO project? There are plenty of warning signs, and here are some of the main ones worth mentioning, which I have come across over the past few years.
Search Optimization 
