I imagine someone sometime ago accidentally, not on purpose, developed there Slide film in the colour negative process and was probably jolly ticked off, however, after calming down found the results to be somewhat interesting!
Instead of the Slide film (E6 process) going into the correct chemical procedure, putting it into the regular (C41 process) film negative process can render some very interesting results.
Firstly, its no longer a Positive, please remember back to holding up slides to the light and seeing a positive image (Fig A) you may have to go back a few years!!
Anyways, since we are scrapping this correct process the film now becomes a negative! ( Fig B)
Now you might be saying, these are in the square format... he's just using his phone!! Alas NO, these are from my Dad's Yashica-D medium format Twin Lens Camera.