At my IL's house, everyone has to spend the early part of the evening sitting chatting/watching TV etc. in the dark, until the timer switches the two lamps on. This happens at the same time of day, all year round, with no allowances made for BST or particularly gloomy evenings.
No one is allowed to have a drink at the table with their meal (except for wine at Christmas), so no glasses of water until a grandchild starts complaining that they're "just so, so thirsty Granny!", whereby MIL will usually relent (reluctantly) and I'll end up pouring glasses of water for everyone at the table except for my PIL - who appear determined to prove that complete hydration can be achieved by simply eating what has been served on your plate!
We are guilty of having a bathroom without a lock - partly because we live in a rented place, so we'd have to get the Landlord's permission, but it doesn't worry us, so we haven't bothered. The rule is; if the door is closed, you knock. It's pretty straightforward and after nearly 4 years here, me, DH and 2 DDs (11 & 13) have all managed to avoid anyone opening the door when they shouldn't. We have the same rule for bedrooms though, so we're all very used to it.
Around 2010, my DM and her STBExH had a couple of friends to stay for a few days right before we arrived for an overnight stay ourselves. After we arrived, I asked DM if she wanted a hand changing the bedding as her friends had been sleeping in the room DH and I were going to be in that night, but she said not to worry, as she'd already done it. Fab. Come bed time, we’d put the girls to bed and DH and I were just getting into our bed, when I lifted a pillow to plump it up, only to find a pair of Y-fronts underneath!
When I raised my suspicions with my DM over whether she really had changed the sheets, she replied "Well, I pulled the duvet down a bit and opened the windows wide, so it's all had a good airing"... I was very glad our DDs were in their toddler beds, so I could be reasonably sure no one else had slept in their sheets too!