I was making soup, and it involved removing some of it to reintroduce after pureeing. Not thinking, I just tossed the cooked mushrooms back into the plastic container they came in raw. After finishing everything in the pot, I tossed the mushrooms back into the soup, and noticed that the bottom of the plastic container they were sitting in was slightly warped. It is a #1 plastic (PET), and the mushrooms went into around 3L of soup. Do I need to just throw out the whole batch?
You should be perfectly fine. PET plastics do leach antimony, and the rate of leaching does increase with temperature, but it would take a much longer exposure time to be of any real concern. You would be putting yourself more at risk by drinking water out of a plastic bottle that had been left in the sun.
Here's a relevant study about water bottles: Antimony Leaching from PET
The abstract adequately addresses concerns to do with high temperatures and PET.
That said, safety is always the highest priority, and if you notice anything unusual about your soup, it's best to err on the side of caution.