Those under the illusion that anniversaries are happy occasions should read the comments of Sir Alex Ferguson last week.
Instead of reflecting happily on two decades at Old Trafford, and before his team paid a fitting tribute with a glorious demolition of Portsmouth on Saturday, he launched an attack on detractors, real and imagined, saying: 'It is scandalous some people think I should retire. It is none of their business. It disgusts me people think this way. It should not be allowed.'
To even think that? When George Orwell coined the term 'thought police', he had weightier concerns than even Manchester United. Ferguson displayed typical bloodymindedness in his assertion that it was his business alone when he should eventually retire; it showed a certain level of contempt for a vast global fanbase that made United for the world's wealthiest club for much of his tenure and the directors who have now mortgaged them to the hilt.
Nonetheless, it was an indication of how accusations of paranoia dog Ferguson in what, on the field anyway, is an Indian summer. Fellow managers have ample evidence of his generosity and helpfulness, but there are others who can testify to very different characteristics.
Admiration for the free-flowing football that has flourished under him - and the remarkable resurgence when many considered his career in terminal decline - are tempered by suspicions of bullying. Just as Ferguson's thirst for success remains unquenched, his wrath remains as fierce as ever in the succession of feuds that have punctuated his reign.
Yet Manchester United's re-emergence as a force in the title race can be dated back to the exiles of Roy Keane and Ruud van Nistelrooy, the latter hardly short of acrimony. Ferguson's decision-making, which long appeared influenced by his refusal to brook anything than approached the status of an internal opposition, now appears all the more dependable.
Because Ferguson stands vindicated. Shorn of captain and top scorer, United have been revitalised. Nemanja Vidic has emerged as towering presence at the heart of the defence; Patrice Evra's speed is starting to outweigh his other shortcomings; Michael Carrick's gradual transformation into a Manchester United player is approaching completion; Louis Saha seems certain to finish the season ranking high among the leading goalscorers; Cristiano Ronaldo, far from the pariah he appeared a few months ago, is threatening to join the pantheon of the United greats if his current, stellar form is a reliable guide; and Ole Gunnar Solskjaer, whose place in history is secured, has made a comeback still more startling than Ferguson's.
Indeed, reflecting their manager, the enduring excellence of United's old-timers, whether Solskjaer, Paul Scholes, Gary Neville or Ryan Giggs, has been a feature of their season. It suggests that the dynasty Ferguson fostered remains in the ascendancy.
Praise has even come from the most unexpected sources. The Scot has been acclaimed as the greatest manager in English history by Arsene Wenger, a compliment Ferguson surely cannot have anticipated, and hopefully will cherish, from his old adversary.
But descriptions like that invite comparison and for a man as intensely competitive as Ferguson, it provides an added incentive; he is at the stage where thoughts turn to rebuilding a reputation in danger of being tainted by underwhelming recent seasons. Adding another Premiership crown or a second Champions League would provide an unarguable repudiation of such accusations.
Indeed, the question had been raised as to whether the upward curve of his early years at United would be mirrored by a commensurate fall in the final times. That question, too, has been answered.
But if Ferguson is still grouchy, it is because such celebrations can provide reminders of his managerial mortality in a business where his longevity and his age (64) can be used to his detriment. The question of the succession has been off Ferguson's agenda since his rescinded his retirement in 2002; instead Keane, van Nistelrooy and David Beckham have been pensioned off.
Despite United's thrilling football, and no matter how imposing Chelsea's extraordinary affluence is, Ferguson still requires another title to validate this team. If not, it will be four years without one and his third generation will suffer in comparison with the first two (which arguably peaked in 1994 and 1999 respectively).
Moreover, with each passing year, the notion gains credence, as memories of an improbable comeback in Barcelona become more dated, that United should have fared better on the continent. That, given the resources and personnel at their disposal in the last 15 years, one Champions League is not enough when Barcelona, Real Madrid and AC Milan have won more.
It is the harshest of criticisms, but Ferguson has always set his team the highest standards and tolerated few who dropped beneath them. So far this season, they have met them. Should they slip up, should Chelsea's domestic dominance and United's European underachievement persist, then the question of the eventual departure of their manager will resurface. But that is the type of thought that Sir Alex Ferguson wants banned.