With the creation of the forums ( over 3 years ago, wow ) it was agreed upon that posting with one character could only be done in one thread at a time. I thought it was a simple rule, designed to ensure continuity and prevent confusion. People have in the past wanted to join multiple threads with the same character. The rule never got added to the rules section because I forgot, but it hasn't come up in ages.
Welp, it came up again because I made an exception that other people didn't feel they were allowed to make. The exception was with regards to writing a flashback (sorry zou) thread in order to expound on a character's history. The rule, along with this exception allowing the development of backstory (technically I suppose it's called an 'external analepsis' ) are now being added.
Also, something I would like to add to the rules and make sure everyone here knows: Rules are negotiable. This is a good example of a seemingly written in stone rule that can be bent. We don't have many rules, so I doubt this will come up often, but if you feel that a rule is hindering your ability to write or play your character to potential, please come to me (or a mod) and request an exception. As a rule of thumb, anything that does not violate "fairness, cooperation, and a consistent setting" is potentially allowable.
We reserve the right to treat exceptions on a case-by-case basis, and change decisions as new information comes to light.