Not sure how probable this is but...
I was just watching a commercial for a show call "The Universe" on the History Channel. It mentioned how scientists discovered "greenhouse gasses" from Venus and its atmosphere. Then I thought, what if millions of years ago, Venus actually supported life; life like our own? Maybe not humans, but life forms that built their civilizations at the expense of their planet. They could have expanded their cities and technologies so greatly that they totally depleted their planet of its natural resources and irreparably destroyed every ecosystem, thus rendering the planet incapable or supporting life. Because of this, most or all life was entirely destroyed and Venus (or any other planet in our solor system) became the harse uninhabitable planet that we know and love today.