Goblet of Fire, mostly through Hermione, repeatedly tells us that the Summoning Charm requires concentration. The following lines in chapter 20 further imply that you need to concentrate on the object that you desire to summon:
Just as long as you’re concentrating really, really hard on it [the Firebolt], it’ll come.
It was time to do what he had to do... to focus his mind, entirely and absolutely, upon the thing that was his only chance. ...
He raised his wand.
"Accio Firebolt!" he shouted.
However, in chapter 27 of Half-Blood Prince Harry successfully summons two objects that he's presumably never seen before ("Accio Rosmerta’s Brooms!") and in chapter 26, the cave reacts to Harry attempting to summon an object that, unknown to him, he's definitely never seen before.
I'm referring to "Accio Horcrux!". Recall from the later chapters that the fake Horcrux does not look identical to the real one. Moreover, he didn't know ahead of time that the locket was the horcrux in question. For all he knew, any of the as yet unknown horocruxes could've been in the cave, so how could he have known what to concentrate on?
In the second case, I can handwave the cave's reaction as it simply reacting to an attempt to use the Charm, but Harry should've realised that it was impossible to attempt the Charm in the first place. As for my other example, what Harry was thinking of is even less clear. As Harry no doubt knows, not all brooms look the same.
Is this an inconsistency in the behavior of the Summoning Charm? As of Half-Blood Prince, Harry appears to be able to summon objects that he can't picture. It gets even worse in chapter six of Deathly Hallows, where Hermione summons some books that she's never seen and doesn't even know the amount of.