Hi Groot, it's pretty much impossible to diagnose any of these problems just from a written description. Please post your 3DM model file with your objects in it that are not working. Maybe you're running into something like there's an error in the model such as a self intersecting area or something similar which is affecting your operations after that.

> Early afternoon it all worked perfect, but in the last hour, it just refused to union them. I
> was able to select both triangles, but clicking on the union button just made MoI go out of
> the bolean menu.

Are you using object snaps when mirroring so you know the mirror is precise? If your mirror axis line is even just a little bit skewed then that can cause problems.

If you can post the 3DM model file with your triangles in it that were not working that might make it possible to give you some more detailed information on it.

- Michael