Email Address Disclosures, Preliminary Report
Since both users mentioned were the last in the list of addresses for the email they received, my money's on a trivial mistake like:    getEmailBody(users[:i])

instead of    getEmailBody(users[i])

I typically prefer a high level of polymorphism in my code/APIs (sensibly handling single inputs vs. arrays) but this is a great counter-example even if not the actual root cause. Every feature is also a liability. Double edged sword. Etc.