what is the hardest situation you can ever face with a client?

From personal experience, when a customer is about to leave you for a cheaper vendor or he goes bankrupt.
I believe it is hardest when you are about to loose your customer in front of your eyes and you cannot do anything about it.

when you give the customer wrong information that would cost him much then that moment when he comes back for revenge then you will feel like ( run forest ruuuuuuuuuuun)

One of the hardest situations is when you are selling a very known, quality and luxurious product, when you deliver a product which has a defect and will take time to replace or fix, spoiling your image and the company's image, and to top it up most of the model line is defective and you have at least up to 20-30 customers complaining on the same issue, and some of the clients actually show up at the same time with the same complaint :-) wuhu party time !!!! Scenario: The client is satisfied with your professionalism and how he was treated and he/she is passionate about the brand.
The product is paid for (huge amounts) and delivered, has departed the showroom, and then therE is a defect in the product(on same day, next day, next week or even first few months after delivery)...a huge defect that can only be changed from the main headquarters in a very far country...and there is no availability in stock for that replacement part and has to be produced in the factory, which can take up to weeks or even months.
In the end the customer is left without being able to use his/her product till the part is replaced, and worst of all they are not even provided with a replacement till his/her product is fixed....
That's really a hard and disapointing and fustrating situation for both the customer and the sales person, you have to really gain their trust back, and manage it very smoothly...

When you give a commitment and you cannot honor your commitment

When the client doubted the quality of the product

When you prepared the presenation for the meeting and at start of the meeting Client ask to present something else that totally related to a close out scenario.

Uncommitted modern morality and ethics

mostly when u did't follow the time dead lines ...

MONEY ISSUES DEFINETLY