First time brewing. I had sanitized everything, but there were a lot of flies around that was attracted to the wort. I fear that they may have landed on my bowl that I was rehydrating yeast with. Or, perhaps it was because I didn't think to sanitize the mouth of my glass carboy -- I only sanitized the interior.
Fermentation got rigorous in the first 48 hours, and it did bubble over a bit before I got a blow-off tube in there. It is currently day 4. At first, it looked okay from the outside, but when I shone a flashlight into it, I think I can see some white spots forming:
Here's a picture: