Axes are not parallel
Not sure if this was from the beginning but once I printed bigger things that I had to split into pieces I realized that sometimes things don't fit together. One issue is certainly that at the edges the slicer produces round corners that add a bulge to the edge. But that can be sanded.
But the big issue is that my printer is not accurate. As you can see at 3 sides I have a gap at one end and the lines that should be parallel are not.
Especially the top line as seen from this image is off, while the bottom line is pretty straight.
What could I do? From the heatbed you can tell which axes are affected. But of course if Z is an issue too I can't test that easily.
Enlarge the image to see it fully. I composed the image from two photos, I measured the distance don't think its just perspective or so.
RE: Axes are not parallel
I just realized that a slight rotation of the bed can cause the illusion of unparalleled lines. But the big gap on the top of the image (at the end of the X axis) is definitely an issue as I tried to put two pieces together with orthogonal walls and there was a noticeable gap of at least 3mm at one end.