Harry Potter Plot Hole?

I love Harry Potter.  I’ve been a fan from the beginning, particularly because I think the way J.K. Rowling so fluidly crafted the series is unbelievably great.  I mean, remember in Harry Potter and the Order of the Phoenix (HP5) when they were emptying out Grimmauld Place and they found a locket that couldn’t be opened, and it turned out to be the horcrux they needed to find in Harry Potter and the Deathly Hallows (HP7).  I mean, she had to plan out the entire seven novels before writing them.

But then I started thinking about how in Harry Potter and the Half Blood Prince (HP6) where Harry tried to get in the Room of Requirement to spy on Malfoy, but he couldn’t because he didn’t know exactly what the room would change into for Malfoy.  Like, in the end of HP5, Malfoy and the Inquisitorial Squad could get into the the D.A. meeting room because Marietta Edgecombe told Umbridge about the meetings and how to get in.

So, why, in HP5 can Dobby the house elf get into the D.A. Room of Requirement when he doesn’t know about the D.A. meetings?  Remember, he got in to put up Christmas decorations for Harry, but Harry never told him what they were using the Room of Requirement for.

Just a brainwave I had, and I wanted to put it out there…