What is the immovable line in my timeline whose only effect is to cut off audio when sharing?

This line looks like the old "out" mark at the top and extends vertically across all sound and picture tracks. The area to the right of it has grey diagonal lines. It had no effect in the editing, but when I exported, the sound drops out to the end. I can't drag or clear it.

Can you post a screenshot?

My software:
Pro Maintenance Tools - Tools to keep Final Cut Studio, Final Cut Pro X, Avid Media Composer and Adobe Premiere Pro running smoothly and fix problems when they arise
Pro Media Tools - Edit QuickTime chapters and metadata, detect gamma shifts, edit markers, watch renders and more
More tools...

I'd like to, I made one, just don't know how to attach it

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This screen shot's better - has the magnifier over the spot.