The worst guest we've had was DH's cousin who came to visit three days(!) after I'd been discharged from hospital after having Baby Wolverine taken out manually.
I was in some serious pain (but I was also on good drugs so that helped a lot) and struggling with bf'ing. We were comp feeding Wolvie (so, as long on breast as she could handle then a formula top-up).
Cousin came over with her two terrors of children - aged two and four - who proceeded to terrorise our cats until they retreated to the roof of the fernery. They then demanded snacks and Cousin got upset when we didn't have anything 'kid appropriate' because "You've got a child now, you need to have snacks for kids to eat!" - please note. Baby Wolverine was less than 2 weeks old at this stage, not so much into snacks, yet.
She then proceeded to give me a lecture about bf'ing and how feeding Wolvie "like that" - ie, topping her up with formula - was going to "make her fat".
When she left (after three hours, when Husband said 'I think Drifter and Wolvie need to sleep, we'll see you at Christmas') Husband went to make Wolvie's bottle and discovered that the tin of formula had been up-ended into the bin, making it unusable.
This was a probably 7/8 full tin of formula that had cost $29.
Husband called his cousin and read her the riot act, and she decided to get sanctimonious about it, saying she had only done it 'for the good of the baby - Drifter's not trying hard enough to bf and having the formula in the house is going to make her lazy!'.
Yeah, we've double-checked the arrangements for Christmas so that we won't be at DH's aunty's place the same day as Cousin and her spawn. As have the rest of Husband's brothers and his parents. I'm proud to say that my MIL actually called her sister (Cousin's mum) and asked her what the hell her daughter was thinking when she did that. I didn't ask her to, in fact it was DH who told her the story in the first place, but I did get a lovely phone call from DH's aunty apologising for her daughters' behaviour.
Aunty is welcome at our house. Cousin is NOT.