Using Floo powder for the first time, Harry fails to follow all the instructions he’s given by the Weasleys and ends up one fireplace off. It happens to be the fireplace in Borgin and Burkes, a shop in Knockturn Alley selling all manner of dark magic items. He barely hides himself before Lucius and Draco Malfoy enter to conduct business.
What exactly went wrong? The films strongly suggest that he mis-pronounced Diagon Alley. However, the passage suggests a number of other things he should be careful about:
"Well ... all right ... you go after Arthur," said Mrs. Weasley. "Now, when you get into the fire, say where you're going."
"And keep your elbows tucked in," Ron advised.
"And your eyes shut," said Mrs. Weasley. "The soot -"
"Don't fidget," said Ron. "Or you might well fall out of the wrong fireplace -"
"But don't panic and get out too early; wait until you see Fred and George."
Trying hard to bear all this in mind, Harry took a pinch of Floo powder and walked to the edge of the fire. He took a deep breath, scattered the powder into the flames, and stepped forward; the fire felt like a warm breeze; he opened his mouth and immediately swallowed a lot of hot ash.
"D-Dia-gon Alley," he coughed. (CS4)
So he actually said "Diagon Alley," and he wasn't warned about speaking clearly anyway. The only thing he's told which would send him out of the wrong fireplace is fidgeting. He is also told that it's up to him to get out of the fireplace when he sees the twins, so he is in control of when he exits the Floo Network. Let's see where he messes up:
It felt as though he was being sucked down a giant drain. He seemed to be spinning very fast - the roaring in his ears was deafening -he tried to keep his eyes open but the whirl of green flames made him feel sick - something hard knocked his elbow and he tucked it in tightly, still spinning and spinning - now it felt as though cold hands were slapping his face - squinting through his glasses he saw a blurred stream of fireplaces and snatched glimpses of the rooms beyond - his bacon sandwiches were churning inside him - he closed his eyes again wishing it would stop, and then
He fell, face forward, onto cold stone and felt the bridge of his glasses snap.
He feels something hit his elbow and he tucks his arm in, which could mean he was fidgeting. He is trying to watch for Fred and George, but he closes his eyes instead. That's when he falls out of the fireplace, and it certainly doesn't seem like he did so intentionally. Something made him fall out of the wrong fireplace. As noted above, the only thing he did which would have caused this was fidgeting. So it was his elbow hitting things that caused the problem. It wasn't his pronunciation of Diagon Alley at all.