In book 5, the Order of the Phoenix, when Harry is captured by Umbridge he tries to tell Snape that Voldemort has Sirius at the Department of Mysteries. Snape supposedly plays dumb with Umbridge and then immediately goes to send a message to check on Sirius. In the meantime, Harry (and friends) travel to London by flying. The book indicates that this takes several hours.
Yet even so, the members of the Order arrived way after Harry. So, are we to believe that it took Snape several hours to message the Order about this? Or that it took them hours to figure out that Harry had gone to the DoM, even after having told them that he'd seen Sirius being tortured there?
Personally, the only reasonable explanation I can even imagine is that Snape took his time passing along the message, or at least the part about Harry having disappeared from Hogwarts. Why did he do that? Did he want Harry to die or what?