Trump has only one enemy he needs to fight: Hillary. The media and DNC want nothing more than to make this election about Trump. Trump needs to make this election about Hillary. Given this fact, Trump's unforced errors here is legitimate criticism. He should learn from Pence and remain focused on the most unlikable democratic candidate for president in our lifetime, not intentional distractions like Khan.
No, he has two other enemies as well.
The Republican Party and the Republican base.
The only unforced error he made that I can see is that he ran for the nomination of a party of backstabbing wonder pussies.
It isn't all unforced. The press asks questions.
And in the Khan case, what exactly is the error? The press has created a fiction that Trump is against Khan the soldier, or Khan the father.
Watch how the press handles the pushback which comes from his surrogates. The surrogates have been pointing at Hillary and Hillary's treatment of Mrs. Smith, calling her a liar. Press reaction? Ignore the point, and repeat a lie about Trump.
Whether we like it or not, this is election is inevitably (at least partly) about Trump.
Heck, the press manufactures issues out of whole cloth, how can a candidate avoid that? See the "People say, 'Trump changed the GOP platform to favor Russia'" line of stories. Those petered out after a few weeks - because anybody who looked at the platform would discover that it had not been changed.
And, FWIW, Trump hasn't brought Khan up in a couple days, and he has continued to criticize Hillary, Obama and the press.
Meanwhile, independent research shows some things about Khan that call his motivations for criticizing Trumps immigration policy into question; as well as calling into question the unstated assumption that he just sort of "appeared" and offered his services to the party.