I hate when you've poured all sorts of blood/sweat/tears/money into an app for a client and worked out all of the bugs they've complained about, only to see them throw their hands in the air saying "I don't know how to sell a mobile app, but since it didn't sell a billion downloads on day one it's a failure". Made more frustrating given that the app is a huge success to the people we've shown it to and selling it is stupid simple for someone with an inkling of sales experience.