Far from the worst customer I have ever had, but definitely the one with a unique problem.
I make custom costume items. My site has a contact form specifying the information I need for a quote: a photo of the item, the desired size and colors if the first photo is not accurate. It is that simple. Except for this guy.
First email, via the contact form:
Customer: Hi I would like an article to be done, I will send you a photo.
Immediately after receiving a second email, with a single link. The link opened to a photo in Google Image Search. Not an attachment of the photo itself, but the search results.
Immediately after, a second e-mail, same result, different image. But these two together were enough for me to at least understand what object he was looking for.
Then I received a fourth email, asking me if I had received the previous emails. All of this within 5 minutes and each mail was sent individually, not as a reply or a chain.
I contacted the customer, made a quote (estimated the size as it was never specified and assumed they are sending the correct color).
The customer then asked for more details, only to mention later that they would need a completely different color scheme. And after 4 emails, confirming the price and all details correct, he let me know that he was only informing him to maybe buy it in the future. Perhaps.
I faced a lot that day.