Perhaps you were wandering around the site and saw a topic with a subject looking like this:

Lab Work [Closed]

Well, it said "closed" but the thread wasn't locked, so you posted a roleplay in there. Suddenly everyone's yelling at you...and you don't know why. Suprise! "Closed" doesn't mean "locked" or "completed!"

Here's the guide to understanding the titles:

[Open] - The person who started the thread wants anybody and everybody to join it! The subject might by edited as his or her wants change, but all people who already joined may stay in it.
[Closed] - A subject that says "closed" means that only people that the thread starter invited may join. Variations on this include "PM invite only," "Invite only," or state the names of the people who are supposed to join.
No tag - Assume that these are open.
[PM Invite Only] or Invite Only - See closed
[AU] - Alternate universe thread. All events taking place here involve a different setting. Characters are the same, but their actions do not mean anything in the real storyline and they may be completely different. For example, in an AU thread, a character might be a fairy on a quest to find magical butter - when in the actual roleplay, they are a tenth grade student cramming for exams. We recommend posting AU when you want to roleplay a character who is dreaming. This tag is also used for writing stories.