I recently tried building a not-so-boxlike house and what did I get? My sims got stuck at the outside corners and complained of not finding a way. It's especially annoying with townies who had to be rescued via MoveObjects. Mind you, it happened in a clean Anygame as well, so I'm quite sure, it's not a custom content problem.
Since google has failed me, I posted this thread. How can I build to avoid this problem? (Besides building normal corners?)

What I've tried:
Placing a column into the diagonal wall with MoveObjects. It helps. I just can't get it to look ok, if the house is on foundation.
Placing a smaller object like a lamp into the diagonal wall. Doesn't work. A pity. Some lamps look rather nice there.

I've never seen this problem and I don't believe that it's standard behavior in the game. There must be something that you aren't telling us about the lot.

If you were to share the lot, we might be able to figure out what the problem is.

There you go! Have fun with the Second Saffron Mansion!
Alright, I'll try to give you any useless information about the lot, that I can think of:
It was built in a NL + base game Anygame, no custom content. Of course, some building cheats were involved, mainly some wild casual level warping. (Note the staircase!) But this has happened on another lot, some time before I understood and used the constrainfloorelevations cheat. Well that other lot was the first Saffron Mansion, which I recreated, because I thought, the lot was borked.
If you faithfully want to recreate the situation I recommend wearing comfy pyjamas and eating Lindt chocolates. :P
Other than that I didn't do anything unusual enough, that I remember it.

Thanks for having a look.

OK, I managed to reproduce your problem. It only occurs on the back left corner of the house, and the sim is complaining about the foundation. My guess is that this is a pathfinding bug in the game, related to the angled foundation.

If I replace the foundation for that one wall with a normal wall instead (using CFE to decrease to foundation height), then my sim is quite happy. Is that a possible solution for you? The outside of the house is basically identical; the only difference might be the alignment of the wallcovering.

Another alternative would be to add some landscaping at that point, so that your sim doesn't get so close to the wall.

What about sticking some plants in and then maybe they would walk around them.

Yes, landscaping the corner worked for me. Note that flowers won't work... they're too low and the sim will try to walk over them. But, the smallest bush seemed to work just fine.

Indeed, this bug occurs only at one corner...
I built a test lot today and had a sim walk around foundations, but I couldn't detect any patterns.
It didn't depend on whether I cut the corners out or added them back in. It didn't depend on the order of the corners. At some corners the sims complain, at others they don't.
The broken corners were usually at the upper right (No. 3 in the picture), but that might have been by chance.
I wasn't even able to fix the corners by adding and deleting or deleting and adding.

The plants would need to be bushes, right? IIRC the sims walk right through flower beds. Well I want to build the lot in a way that the sims can tend all the plants, so sticking a bush on the corner tile is out. Of course, planting a few bushes around the corner tile would probably work, but how do I do this without it looking kinda odd?

Well replacing the foundation with a regular wall should work well for brick foundation. Remind me to stay away from lattice foundation!

The whole thing just remains kinda weird. And why am I the first to stumble across this?

Thanks for your suggestions!

Edit: Oups: I didn't see the last post. (Should reload the page once in a while...)
So do you guys mean putting a bush onto the corner tile or somewhere else?

"Upper right" is meaningless without knowing the rotation of the lot, which people often describe in terms of the direction of the sun. If all of your lots have the same rotation, then it makes a lot of sense to me that the problem would always occur at the same corner.

Yes, I added the bush to the tile that was half covered by the foundation, using moveobjects.

Another potential solution, if you're willing to use CC, would be to install an invisible wall on the corner. I believe that Numenor has a set of walls which includes an invisible wall. Basically, you just need something that forces the sim to walk around that tile.

My 2 cents:
Firstly, putting a shrub into the half-square as Mootilda did should indeed work, since Sims don't actually enter the square containg the shrub to trim it, IIRC. Try it.
Secondly, putting a shrub into the half-square and surrounding it with other shrubs won't really work because the half-square shrub won't be accessible for trimming.

Or, you could put in flowers and low-level fencing the Sims can't walk over.

Odd bevavior, which I too never noticed.