Voldemort's plan to get Harry into the Ministry in OOtP didn't make sense. Voldemort was giving Harry the dreams about the Department of Mysteries for months, hoping Harry would get curious and go there. That in itself doesn't make sense. Did he think Harry would just go off to the Ministry after Charms or something? How did Voldemort think that Harry would be able to go?
"Dumbledore never told you?" Malfoy repeated. "Well, this explains why you didn't come earlier, Potter, the Dark Lord wondered why-"
Order of the Pheonix - page 786 - Bloomsbury - chapter 35, Beyond the Veil
Then, when he gives Harry the fake vision about Sirius being tortured, how did he think that would work? When Harry gets it, he immediately tries to tell McGonagall, but she's in St Mungo's.
He waited for the second when the old man's heels disappeared over the treshold into the Great Hall, then ran up the marble staircase and then more staircases toward the hospital wing, hurtling along the corriders so fast that the portraits he passed muttered reproaches, and burst through the double doors like a hurricane, causing Madam Pomfrey, who had been spooning some bright blue liquid into Montague's open mouth, to shriek in alarm.
"Potter, what do you you think you're doing?"
"I need to see Professor McGonagall," gasped Harry, the breath tearing his lungs. "Now...It's urgent..."
"She's not here, Potter," said Madam Pomfrey sadly. "She was transferred to St. Mungo's this morning. Four Stunning Spells straight to the chest at her age? It's a wonder they didn't kill her."
"She's...gone?" said Harry, stunned.
Order of the Pheonix - page 730 - Bloomsbury - chapter 32, Out of the Fire
The only reason he doesn't tell any Order member is because he doesn't think there are any left to find. Umbridge is the reason why Dumbledore and McGonagall are gone, so it was just a fluke that Harry didn't get to tell them. If they had been there, Harry would have told them and they either would have immediately found out that Sirius was okay, or they would have gone to the Ministry themselves and left Harry at Hogwarts.
Plus, he only was able to even get there because he, Hermione and Umbridge went into the forest. All of the factors that led Harry to the Ministry alone were flukes. How could Voldemort possibly think he could get Harry there alone under any circumstances? There was a 99% chance no one would show up because they'd find out Sirius was safe, or that the Order would have shown up without Harry, which would have been no use to him.