Can be bad. Worse when you go back out with it fogged and the condemnation freezes. To avoid it, keep your camera in your bag when you go inside, at least 15-30 minutes, but it really depends on the temperature difference as well. You just want to warm it up as slowly as possibly, so the deeper in a bag, the better. Silica gel packs are great, but if I forget them or I only have a small setup (1 body, 1 lens, or I used to this with a large point and shoot), I wrap it in a t shirt, put that in a ziplock, then throw that in my backpack. Worked fine for me. I have also had a lens fog up before. It isn't good to get in the habit of it, but I let it sit for a while and it was fine.