Shape bender plane problem

[dohyungkim]

I did shape bender flat one to curved one behind in the image

I don't know Why I got that diagonol lines in the red circle.

Can anyone tell me how to do shapebender without that line???

by the way I couldn't delete that line without losing plane....

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[TIG]

It's because the bending skewed that particular surface, so to retain it a diagonal was added.
The two triangular facets are no longer coplanar, so you can't simply delete that shared edge.
You can use Erase + Ctrl to smooth it.
If you use View > Hidden Geometry > ON you'll see various added smoothed edges in the bent form.
Somehow that one slipped through.
So just 'smooth' it, and carry on...