Hi Olivier, often times when you do a sweep along rails that are different lengths, as the profiles slide along the rails they will bank a bit rather than staying as vertical planar sections, that banking will then introduce the type of crease in the end shape which then becomes visible when mirrored.

There is a sweep option called "Maintain tangent" which can help with that, it's not always able to be enabled but you might try that. See the documentation for sweep for an example of that exact mirroring problem: http://moi3d.com/3.0/docs/moi_command_reference7.htm#sweep

Another thing that can tend to help is to build your surface as a complete shape from the initial sweep rather than only building half and the mirroring. If you're having problems with that can you please post the 3DM model file so I can take a look at your particular setup?

In general when you have a question about a particular problem it's very helpful to post the 3DM model file with your curves in it instead of only a screenshot, so people can examine your curve setup in more detail like zooming around them and inspecting them and repeating your construction steps, none of which can be done easily just only looking at a screenshot by itself.

- Michael