I wonder, what actually happened to my sim, never experienced it before.
So, she was a writer, mostly stayed at home, lived alone. One winter night I couldn't find her in her apartment, though I remember I didn't send her anywhere out off the lot, and she had some actions in her queue, all of which suddenly disappeared. I double clicked on her and found her on board of a taxi boat that took her around the island (it was in Ethan Drift world). I didn't even order her to go anywhere, wanted to see what would happen next. The boar made a circle around the island, she came off the board on a beach, was in her swimsuit, and had a "polar bear" moodlet for swimming in winter, lol. Is this a random action from Seasons EP or what?
Strange enough, this happened to her again in late spring, with no moodlets this time.
What was that?

Is your lot close to the beach/ocean? Sometimes Sims spam the "Swim in Ocean" autonomous interaction and run off on their own to swim. It could also be a fantastic example of EA's pants-on-head moronic routing; maybe the Sim decided to go home and EA calculated a stupid route for her.

Alternatively, do you have Autonomous Fun In The Sun installed? I used it for a while but had to take it out because Sims would run off to swim in the ocean the minute they didn't have an action queued up.

If it helps, I recently directed a Sim to do her laundry. She took it out of the hamper and ran to the other side of the island, in spite of the washing machine being literally next to the hamper And here are more examples of great routing decisions:
The quest for spotlight mushrooms
Dumping a toddler on a bridge and diving to retrieve it
Circling the block to go to the gym across the street
Walking further to take the subway rather than walking to the destination
Likewise

Thanks for the reply; so, it's a bug, not a feature. Sims do choose stupid routes from time to time, but that was just too extreme. I didn't even send her out of her apartment. So, this was probably her route from one room to another, XD

Do you use NRAAS Story Progression? When switching households in the past I have seen sims being pushed here and there while I was controlling another household and switched back - because 'push to lot' was active in SP and was controlling the sim while I wasn't. Sometimes things are already queued up when you switch so they don't get cancelled right away.