![Image](https://forums.factorio.com/images/ext/8b36122e48cfb70fbf751cb41fbb7328.jpg)
This happens at random spots in my factory, for no apparent reason. I've had it happen 3-4 times so far since I started with 0.16.
Provided I remember correctly, in all cases it's been underground entrances being sideloaded directly from splitters. Edit: Not true - had another instance where a belt was sideloading the underground entrance.
I'm using no mods.