That's a camera error. If you processed it in a film chain, any issues would be running the length of the film not across it. Plus if you look at the sun on the right it's the same while the people are darker.
I've had a slide mounted incorrectly out of the 10 rolls I've sent them over the years. Even that one wasn't bad you could sort of see the sprockets. I tried to ask for unmounted slides when I sent a roll through walmart but the reply was "sorry, we're set up to mount them" or something like that. IF you send them directly will they forgo the mounting?