Trump claimed that his inauguration opening concert at the Lincoln Memorial was a rare way to begin an inauguration when the facts say the exact opposite.
I can't say I give a damn whether Trump was factually accurate about words were spoken by his predecessors from the grounds of Lincoln Memorial. Indeed, were it so that Trump has been wrong about contextually unimportant details I wouldn't be of a mind to deride his integrity. What does bother me is that Trump, as so often he is wont to do, made a statement that can be called into question and it was a claim that didn't have to be made.
I suspect that nobody really cares how often presidents have made inauguration addresses at the Lincoln Memorial, yet we know that at least in recent history, their doing so isn't uncommon. What presidents between 1922 and the 21st century have? I don't know. I don't care. Fifteen presidents have had the opportunity to make such a speech, and now at least three (25%) have. I know that if Trump didn't know who other than Bush and Obama have done the same, he could have made his claim with an appropriate qualifier.
But as I said, the incidents of speeches from the Lincoln isn't the point. The point is that Trump, far too often for someone who's a leader in general and a President in particular, makes statements that are factually inaccurate. Simply fact checking what he thinks to be so prior to making the assertions/attestations. It's human to forget or know minor details, but it's irresponsible, immature, disingenuous and dishonest not to give credence to the fact that one is human and that there's an element of uncertainty regarding a given claim one is making.
That is now and continues to be the problem I have with Trump. Adding insult to the injury is that the man has presented himself as a populist leader. Not as an expert, though he must now become one on everything he discusses. If anything, however, populists are afforded the luxury of not knowing "everything" so long as they are honest about it and do something to know "inside out" that of which they speak and decide. A populist has considerably more leeway than would an experienced public policy player, but not so much leeway that s/he can ignore the basics of simple honesty and integrity.