Tested DS after school; running high all day, BG was 266. I gave him a 1 1/2 unit correction, then he decides he's going outside and play basketball. Comes inside and says he feels low. I test his BG, 40! I asked him if Dexcom buzzed, he said no. While he's shoving Halloween candy in his mouth, I check Dexcom, it says 108. I try to calibrate it and this message comes up "Enter BG in one hour". It's showing two 1/2 full hour glasses and a blood drop and won't allow me to enter a BG reading; in fact there is no number at all. I know I shouldn't have calibrated when he was low, I've just never seen that message before. I guess he was dropping so fast that Dexcom couldn't keep up.