If Sirius had told Harry EXACTLY what the package he gave him was, none of the later plot of OotP would have happened...

instead of just handing harry a wrapped package and telling him to use it if he needed him, all Sirius needed to do was to be more direct, "this is a two-way mirror, you and I can stay in touch a lot easier. If you need to talk to me, you can reach me using this." Then harry could have easily checked to see where Sirius was.

this and other scenarios in HP could have been prevented if people were better with communication and less being vague/mysterious.