Both my worst house-guest stories are about my MIL.
The first one was when she came up when DD#2 was 4 weeks old, DD#1 was 2.5, and we were packing up to move to a bigger house. She said she ought to fly back the first night (she does not travel well), and then she got sick... Seems to have been an inner ear infection, or Bell's Palsy, but she woke up in the evening and she couldn't move one side of her face. Rather than come tell me - I was nursing in the living room, while DH and DD#1 were asleep - she waited until the morning when DH was awake. We took her to the hospital, and she stayed there for a week. Then she came back and kept on getting worse - I would help her get up from her bed to the bathroom, think of things to make her she might eat, and otherwise help her out (DH was at work), and try to keep the toddler quiet (it was a very small house, and did I mention I'd just had a baby?). She went off at me this one time for not changing her sheets - well, she was never out of them, and she could just ask... Eventually we got her back to the hospital, she stayed for two weeks, we moved, and then she came to the new house. Fortunately, she stayed until she could manage stairs, and the hospital did supply a home-care worker (I didn't have to tweeze her chin, thank goodness). She would come downstairs, and sit and glower in the living room while I fed her, fed the toddler, nursed the baby, tried to eat myself, unpacked some, and tried to finish a project that I'd accepted (self-employed). She held the baby once, made one "meal" (put cream cheese on a rice cake for the toddler), and told my mother that she was surprised I hadn't unpacked more of the house. Two weeks later, DH took time off work and flew back home with her. I arranged for a home care nurse, and help at the house. A few months after that she sent a long letter about what a bad mother I was, as my toddler had (gasp) a tantrum while she was there.
We invited her up again.
DH used the "be a better person" line.
She, my mother, and my brother came up for Christmas 2005. MIL stayed with my mother a night before the plane trip We moved the girls into our room, and put a grandma in each girl's room, next door to each other. MIL complained that she had to get out the wrong side of the bed to go to the bathroom.
Christmas eve, my mother heard loud mumbling coming from MIL's room at 4 or 5am. Knowing of the previous health problems, and being a nice person, she knocked on the door to find out if MIL was OK. MIL started "ranting" (my mom's word) at her, about how she and her clan had ruined my MIL's life and future, and how she had never liked me and I was raising her grandchildren all wrong and it was my mother's fault and on and on. She nearly hit my mother, and when my mother left the room she went after her to continue the rant. Eventually, this woke DH and I up, and DH went out to tell her not to go back to bed, and not ruin Christmas for the girls.
Fortunately, the girls didn't wake up, and we basically separated the rest of the stay - MIL and DH by the fireplace in the family room, and the rest of us in the living room. DD#1 was sitting at MIL's feet at one point, and admired her necklace, and MIL said it was a family heirloom, it had come to her, and it ought to go to DD#1 as the first child of her first child - but it wouldn't, because DD#1 didn't have MIL's birth last name (she has her father's last name, the last name of the man MIL married). "Do you like it?", MIL said. DD#1 said no. Fortunately, I'd prepared the children by saying that MIL was rather old, and not well, and she gets a bit cranky sometimes especially when it's noisy, "but grandma still loves you, of course."
MIL and my mother flew back in the same plane, but different seats. My mother didn't sit by MIL in the waiting room, which caused MIL to rant a bit more at her in public. She also called a lady who asked her for advice on flying "common". We had called MIL's cousin, who MIL was going to stay with after staying with my mother for a few days, and she met MIL at the airport and took her home. She quite understood, as she had stopped talking to MIL for 10 years sometime previously. My mother has not talked to her since.
In deference to my husband, we stayed with MIL for 3 days last summer, but it was about 2.5 days too long. We are not doing that again.