Facebook and Twitter, now you've done it, even the pope is sick and exhausted of people whining all of the time.
The sign, which is written in Italian says, "Violators are subject to a syndrome of always feeling like a victim and the consequent reduction of your sense of humor and capacity to solve problems".
The sign is adorned with the worldwide symbol for "no" - a backslash in a circle.
"To get the best out of yourself, concentrate on your potential and not on your limitations", the sign states.
"So stop complaining and act to change your life for the better".
Motivational speaker Salvo Noé presented the pope with the sign last month, reports Vatican Insider, which first posted the image of it. Francis promised that he would post it outside his apartment, which doubles as his private office.
Francis has had no shortage of complainers in the Vatican and in the Church since his election in 2013.
He has also encouraged the world's billion-plus Catholics to be more joyful.
Mueller, who disagrees with the pope on a number of issues, apparently expected his mandate to be renewed automatically.
Mueller later told a German newspaper that the way he was treated was "unacceptable".