To avoid spoiling people of events they may not have known about in a series, Cosplay.com has a spoiler tag you can use.
The tag is [*spoiler*]enter your spoiler here[*/*spoiler*] without all the * parts in it. ^_^
Here's an example with a made up series:
I totally loved episode 52 of Magical Cornchip Avengers!! But it was so sad when
and I could barely stand to watch anymore!
SPOILER: (highlight to read)
Captain Cornnuts died trying to save the troops
Basically, anything special that is not known to the public is a spoiler. Sailor Moon's identity as Usagi Tsukino isn't a spoiler, for example, because we know that from basic series synopsis and promotional art without even having to watch the show.
Someone dying, someone's identity being revealed later in a series (basically something you wouldn't know from episode 1), someone's secret being told that you didn't learn til way late, etc, are spoilers. Even if you think it's common knowledge, please try to be courteous about it and don't assume for others what you think as nothing serious.
Please save the members a lot of heartache and try and put this tag to use when possible. Thanks!