Maybe he just put the wrong sticker on there. That's not a warning sticker, it's the one they apply to visas that need to have work permits associated with them to get extensions.
You can get there as early as you want. They won't open the gate til they do, and won't begin processing til 8 or later. Mondays can have hundreds in line by the time they open. Fridays will often be empty at 9 (it once took me thirteen minutes to drop off and be done on a Friday).
I'd get it ahead of time if you're cutting things short. I almost missed a flight to Paris getting one on the way once. Fortunately, the IO's 10-year-old daughter was there to help him with the crowd :-D