Very prevelant. Most of the time, clients hire consultants because things aren't rosy. The biggest barrier to change is people, either blocking change by behaving badly or refusing to go along with a boss who is behaving badly.Usually, any unethical client behaviour is a feature of their internal politics and something you generally avoid getting involved in. However, if you're a senior seller you may choose/have to play off these politics in order to get in with the right consultancy buyers.In terms of clients behaving unethically towards the consultant, there are plenty of examples. Not least, is the common situation where the client treats the consultants without respect. This is the "I pay for your time so you're my slave" attitude. This is often passed off as normal, but at the end of the day there's a strong argument that treating anyone in the workplace without basic interpersonal respect, from the CEO to the cleaner, is unethical.Another example comes when clients put consultants in untenable situations, requiring them to implement unethical decisions or feeding them misinformation to force the outcome of the consultants' recommendations towards unpopular decisions that the client does not want to be seen making themselves.A third example is where clients invent or purposely create problems in order to abuse the rework option and demand work outside the agreed project scope.Then there are those clients who demand extra work, but never quite get around to signing off the change of scope agreement and subsequently refuse to pay their bills.This isn't meant to sound like consultant/client relationships are all in the style of "us vs. them". You have to give clients leeway and understand that, if they had high calibre, smart, motivated, engaged, honest, hardworking and professional staff on the case, they wouldn't need to pay for outside help. Part of the client need is to have consulting staff come in who model the behaviours the client wants their own staff to adopt.Even when the client is being a tw*t you have to remain professional. But part of the consulting role is to offer advice proactively when we spot a need. Sometimes that advice has to be, "your behaviour is compromising the value and quality of your organisation, please leave or we won't continue working with you".