When email app Mailbox launched in February and instituted a wait list to usher in new users, the company unleashed the hellish fury of hundreds of thousands of people left waiting for the app. Mailbox’s controversial launch seemed to signal a clear warning to upcoming startups that digital wait lists will incite angry tweets and blog posts — bad PR, by most standards — but the wait lists show no signs of stopping. Calendar app Tempo launched a week later with a wait list and people pulled their hair out. Last week Loom, a cloud photo-storage app launched with a velvet rope as well. The bitter tweets followed, just as they had before.