I was just chatting with a girlfriend about this topic, and she asked me what was the biggest thing I stuffed down there. I had to think hard about it. I remember once had a small mailing tube stuffed down there after checking the mail at the office, and it stayed there for most of the morning. Nobody said anything! Finally, I told one of the subs on the job, "oh, this came for you." I didn't think twice about it. And I didn't notice any reaction. It was rather warm though.
That is where I stash my tape measure when we're estimating. Had a laser level in there once, too.