I worked in a transmission shop in high school and I saw a LOT of pans like that...to be honest...all of them. Every single pan that I dropped had metal in it...don't stress it unless you are already having problems. If you haven't changed the fluid in 50k miles then changing the fluid might actually bring some of the wearing or failing parts to the forefront, especially band servo pistons. The new detergents go in and clean out all the gunk and can be the death blow to an ailing transmission.
Regularly change your trans fluid as per your owners manual...if you don't and let it go 50k miles then you might prepare yourself for the consequences of ignoring your maintenance schedule.
You have to think on the positive side...all that stuff is not circulating through your transmission and the magnet has done its job.