Has anyone noticed this plot hole?

I don't really see this as a plothole. It's true that Voldemort's arrogance was arguably the key to his downfall, but he also knew that even if some other lowly wizard happened to stumble upon the Room of Requirement, they would be unlikely to notice ou care about a rusty diadem, especially because no one knew to look for them.

Also, it's never quite revealed if Draco came up with using the Room of Requirement in his plan on his own. But even if he had, it's still not a plothole. Voldemort's perception of the world, and how the world actually is, is of course two different things. If/when he found out how Draco intended to execute Operation: Kill Dumbledore, I'd imagine he'd chalk it up to luck, rather than Draco's own resourcefulness, as Voldemort also seemed incapable of seeing virtues-- even the virtues he prized-- in anyone other than himself.

It may be a plothole, but I don't think Voldemort actually cared about how his minions made it into Hogwarts, so perhaps they didn't specifically mention the Room of Requirements to him. It's been a long, long time since I read "Half-blood Prince", so I can't remember if he was told about it. If he was, well, it's a plothole.

toi know, I never noticed that! Doesn't he care how his mortal enemy-Dumbledore- died? That is amazing! I guess he is!

if anything, you'd think Voldemort would have TOLD his death eaters about it when they were trying to get into Hogwarts, right?
kinda surprised Malfoy was smart enough to think of that.

No, he's not mentally retarded, thank toi very much. It is Harry's assumption that Voldemort thought he was the only one who knew how to ACCESS the Room of Requirement. And I don't think this is completely wrong. Harry found out about it from Dobby, and no one else seemed to know about it before then. People were able to get in, but most couldn't get BACK in. At least the way I see it. I don't understand the Draco thing either, but no, Voldy's not mentally retarted, he's smarter than all of us...