I am very thankful I have never had a bad wedding client, and more importantly, that every wedding I have ever shot was well-received.
The good news for the photographer is that she will not lose if it goes to court. They hired her and she performed the work. No reasonable guarantee can ever be made to a wedding client that they will like the shots. The best you can do is show them an honest portfolio that shows your range of work to give them a general sense of what to expect.
Years ago I second shot for a photographer who asked the bridal party to jump for a shot in a field. When they landed, their heels dug into the mud. I have shot a wedding where somebody got right in front of me during the first kiss. I didn't get the shot. I shot a wedding where an elderly gentleman stumbled and nearly knocked me into the cake. That would have been disastrous. Another time I second shot for a guy who spilled salsa all over his pants and had to shoot the rest of the wedding looking like a fool. One time I tripped over a pew during a church ceremony, only to recover and then trip over another one, narrowly escaping a pretty bad fall. Fortunately, nobody saw or heard me trip from what I was told.
Things can go wrong for any photographer, and I've seen it happen a lot. I would hate to see any of these things ever backfire on somebody because most wedding photographers I know work their asses off and have a job that is incredibly demanding creatively, physically, and socially. We can't show stress, we can't lock up, and we have to be genuinely happy so that those around us are happy. Sometimes this is difficult.
I had a wedding in the city a couple years ago on a day when there was a blizzard. I woke up in the morning feeling sick. I threw up ten minutes later. Then my ride showed up and we barely made it there because of the snow. Then I shot outside for a good portion of the day, feeling like I was going to die. Then, after twelve hours, my ride told me he may not be able to make it back. What a shitty day I had, but with regards to the wedding itself I was ecstatic to be a part of their day. They were lovely people and to this day they would not guess that I was sick or uncomfortable that day.
I've shot weddings where there were big fights amongst the family. Talk about socially demanding! The father of the bride came up to me while his daughter was about ten feet away. "Look at her," he said. I replied "She's such a sweetheart and she looks stunning." His reply stopped me in my tracks. "No, she's a bitch and you know it." I don't remember how exactly I replied, but probably a nervous laugh followed by awkwardly backing away.
I shot a celebrity wedding that was on television and it was much more stressful than most others. There were a ton of production guys there and the whole crew was on edge all day. Reporters were there as well, and one press photographer stood in front of the groom at the ceremony, blocking his view of his bride coming down the aisle. Other people were making my job very difficult for me. I was stressed out all day and was really hoping it didn't show.
The following day I got an email from the bride saying that her day was very hectic and many people were stressing her out but seeing me brought her the calm she needed. I was both shocked and relieved to hear this.
As far as unhappy clients go, I have had three of them that I can remember, and their unhappiness came from not wanting to pay, not because they didn't like the work but just because they're the sorts of people who complain at restaurants in order to get a free meal. In all cases they had already been using my shots for commercial purposes prior to their refusal to pay.
If I ever got a wedding client like this, somebody who wanted to get money back just for the sake of it, they would certainly have some ammo, or at least be able to make something up. "He nearly knocked over our cake!" "He took a selfie with a guest!" "He was taking photos on guests cell phones instead of his professional camera" (both things I am frequently asked to do and always oblige unless it's a bad time).
My best advice to others is to vet your clients. When you meet with them, look at it as a two-way interview. They want to decide if you are a good fit for them, and you want to decide if they are right for you.
Bad clients almost always try to negotiate on price, but it gets tricky because plenty of very good clients do the same, and there's nothing wrong with that. Bad clients ask a ton of questions and make a lot of demands, but some good clients do as well. I had one couple ask me to fill out a questionnaire they wrote up. I thought it was a red flag but they turned out to be absolutely wonderful and I keep in touch with them to this day.
A bad client feels entitled to you. A bad client talks to you as they would any other service person. You are to be there at their beck and call. A great client makes you part of their wedding day. They treat you like family. Their parents stuff your face full of food and everybody tells you how great you are and how much of a pleasure it is to work with you. A simple good client is respectful and professional but doesn't necessarily treat you like family.
Avoid anybody who makes you feel like a servant. Contracts don't mean much to clients like these. Some people live their lives suing people and complaining for a profit. I know somebody who got a free honeymoon because they complained about every little thing along the way. Joke is on them though because they spent so much time complaining that they never got to enjoy a nice, relaxing honeymoon.