Hi Steve,

> How does that happen and how do you repair it?

It looks like that happened because your 2 surface halves don't meet up exactly - there was a small gap between them but the gap was less than the join tolerance so it was still possible to join them together anyway.

It is possible to have gaps between the ends of edges as long as they are within a tolerance - in this case if you draw in a line you can see that the gap is something like 0.0003 units in size, so it's small enough to be in tolerance.

If you turn on surface control points you can see that the 2 surfaces have a bit of a gap between their control points.

So possibly the surfaces were not totally flush to the mirror line that was used to mirror things over - either the surface was not quite on the same line or it was angled slightly or the mirror line was angled slightly or something along those lines.

Anyway, that's what has caused that little gap - if it's an even gap and not because it's angled it can be fixed by using Edit > Separate on the pieces and then select all the trim edges on each piece and hit delete to untrim them and go back to the original full base surface, then position those so that they are totally flush with one another instead of slightly apart, and then rejoin.

Let me know if you still need help or more information.

- Michael