Hi dave, something like that usually means that there is some error in the trimming boundaries, something like the UV trim curves do not touch each other end to end or 2 edges criss cross over each other.

When that happens on a more complex surface you would usually need to "untrim" the surface and then retrim it with the surrounding surface's edge curves. There is some information on untrim here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

And there's more detailed information on this kind of low level surface repair process in the object repair tutorial here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

But for a simple surface like you've got there just deleting it and reconstructing it as others say above would do the trick.

To delete it you need to do a "drill in" selection to select the face. That's where you do 2 clicks on an object, the first click selects the whole object, and then if you click a second time after that on the same object you will "drill in" to select either face or edge sub-objects. You want to select the face and hit delete.

Then you can use either Sweep or Network to reconstruct the surface. For Network select all 4 surrounding edges and run Construct > Network. For Sweep select the 2 short curves to be profiles, and run Construct > Sweep and then select the 2 long curves as rails.

Once you have built a new surface there, select it and the surrounding object and use Edit > Join to join it back into place.

- Michael