I fixed a subtlish bug in Mind Traffic Control today.
I was trying to pull the email address out of user objects using user.email() ... however it seemed that you might be able to have users who don't have an email address and for who user.email() fails or returns None. Not common, I guess because user id's are based on Google accounts (which is mainly Gmail accounts I'd assume.)
Anyway I've now wrapped all attempts to get user.email() in a try. But next question, what should I do instead? When I find a user without an email, what should I use to identify him or her?