Let's say after working all day, making dinner and congratulating yourself for being so clever as to have frozen cookie dough at the ready, you bake cookies. And you pull the cookies out too soon. Let's say the middle is gooey; not the good kind of gooey, but the soggy and awful kind of gooey. And let's say, hopefully, desperately, you let them sit for five minutes to firm up and they never do.
Also, keep your broiler at approved temperatures for your silpat. My broiler defaults at 550 degrees but my silpats can only handle up to 450 degrees. Make adjustments as necessary.