Jesus himself equates gossip with major mortal sins such as murder and sexual immorality.
“For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy,slander, arrogance and folly." - Mark 7:21-22
Speculation is accusing someone of sin when you aren't sure that they're guilty, or guessing at what really happened when you only have a few pieces of evidence. It can be deadly not only spiritually, but also in real world terms. A case in point is the boy Reddit users accused of being the Boston Bomber. He has recently turned up dead, pulled from the waters off the coast of Rhode Island.
The cause of death for that boy has not been determined, and his name has now been cleared, but the anguish and the grief caused by these false accusations for him and for his family cannot be measured nor repaid. If it turns out that this boy was murdered because they acted on the speculation of the Reddit community, those who were involved will be guilty of participating in his murder. They may not have directly committed the murder, but they have born false witness against their neighbor and are indirectly responsible because of it. That they believed it was true does not lessen their culpability.
This is one of the greatest dangers of the internet. Speculation can run rampant, good men's names defamed in an instant, character's detracted, and the flames of hatred and bitterness fanned by wagging tongues. Let us all use caution when speaking of the actions of another, especially when their actions may be suspect.