Now, this happened with Morlock once, but on that occasion, I saw him right away after hearing the noise, and dove to grab the bag before he got away. It was easy.
Setzer ran downstairs into the basement, knocking a few things down as he passed them at top speed. He finally stopped under a chair, breathing fast. I spelled cat pee, and wasn't sure if it was new or old. Setzer tends not to pee in inappropriate places, so I was pretty sure it was fresh. And it was. Apparently, he'd wet himself in his terror. I tried to pick him up, but his claws were wrapped around the metal chair leg. He didn't struggle when I tried to separate his paws, just stayed there stiffly.
I met up with Chris upstairs, and he removed the bag while I held Setzer up. The bag was pretty shredded, and came off easily when removed with human hands, but still wouldn't have easily come off for Setzer. I felt badly about the next decision to make, but it had to be done. Setzer needed a bath, since he'd wet himself. So, back-to-back trauma for the nice gray cat. I gave him a good rinse where he seemed wet, and got a few claw marks along the way. I'm glad I keep his claws fairly trimmed, or this would have been even more painful for me.
Afterward, Setzer's tail was wet, and non-fluffy. So strange!