In your first example, there's no continuity on the curves chosen for fillet. You can see that by selecting the curves you want to fillet (the ones in yellow in your example image) and copy them to the clipboard. Then hide everything in your scene,
paste your curves back into the scene, select them all and press join.
The result will not be one single curve.
From there the problem in your filleting I think...

Pilou, yes I got filet to work on the first level too, but in a somewhat surprising way, by increasing fillet radius. (And by selecting just two of the circumfering edges I got that weird bug with the fillet sticking out from the object)

You're propably right Schbeurd. I don't think there should be a reason for the edges to be discontinous, though, as everything is modeled with snap to grid. Also I can't find any micro edges or surfaces that mess it up.

Something appears to have gone wrong with the trim, here's a zoom-in on one corner.

Yes, I just verified it - you didn't model anything wrong, trimming those surfaces by the curves has a bug where it doesn't do a clean cut right near the end where there is tangency.

It looks like the mechanism that projects curves on to surfaces doesn't deal with grazing tangent conditions that well (which tends to be a quite difficult area of calculation). However, there is a proper handling of these things in actual surface-surface intersection rather than curve projection. So as a workaround, I extruded your trim curves out (using both sides option) and used the extruded surface as the cutting object. this then generated a good cut. If this seems to be a recurring problem area with projected curve trimming, I may need to internally switch to this to an extrude and surface-intersection method instead of a projection method.

Anyway, I've attached a model using the surface trimming workaround, it behaves well now.

- Michael

Looks like you're right.
Testing a few times now it looks like the workflow I described will always produce a bad trim on the first try. *However* Hitting CTRL-Z and then redoing the trim always seems to fix the problem. I've done this 4-5 times with different files now.

1: turn on snap to grid.
2: draw a dome-like curve
3: extrude it
4: create an arc for trimming the corners (Like in the file I supplied earlier)
5: do a trim of the extruded object with the ark

You now have an object with a malformed trim at the corner(s)

6: Hit CTRL-Z
7: Redo trim (Not CTRL-Y)

You now have an object with a good trim at the corner(s)

(Edit: looks like we both found a workaround at the same time:-)

> You now have an object with a good trim at the corner(s)

Hmmm, I was able to repeat this over here too. That is quite odd.

Maybe it is a clue that some setting on the object (such as a tolerance value for an edge) is not set properly and gets fixed somehow when the object is restored through the undo process....

- Michael