Basically, a spoiler is any piece of information about an upcoming episode. Spoiler information can range from things as major as the fact that a character is going to die to something as relatively mundane as the fact that a certain seldom-seen character is going to make a surprise reappearance. Even if you were to leave Carter's name out in that particular example, the mere knowledge that any of the characters is going to get hit by a train constitutes a spoiler for most people. The generally accepted Usenet rule is that once an episode airs, spoilers don't apply to it. The generally accepted practice on alt.tv.er is once an episode has at least somewhere in North America, the spoiler warnings can come off.
People living on the west coast, avoid the group for a few hours.
Many thanks to Alan Sepinwall for that little lesson. It's almost inevitable, and is often quite enjoyable. Those who revel in OT posts are then free to revel at will, which should be done unharassed by those who don't. Since you're reading this, you're already ahead of the game. Read the group you want to participate in without posting for a while.