Various Trap Doors (poss spoilers)
I just read again some of the explanations JMS gave for "trap doors" especially the ones related to the Susan Ivanova character.
Supposedly Lt Cdr Takashima in "The Gathering" was supposed to be a deep-cover traitor/mole, and when Tamlyn Tomita didn't return for the series, Ivanova took over that role and position. And THEN when Andrea Thompson decided to leave the show, the traitor/mole bit was shifted to her Talia Winters character who could then be discovered and leave.
But it doesn't really hold together, because not only was there no real portrayal of Takashima doing anything wrong or deceptive or whatever in The Gathering, there was also none by Ivanova in the following episodes when she supposedly took over that position.
The description of Sheridan's mentoring of Ivanova is very similar to Sinclair's mentoring of Takashima. (Served together on Mars, etc.) But even if Ivanova's "real mission" was somehow delayed until Sheridan arrived to allow for providing the same description of their background, Ivanova wasn't doing anything underhanded after that, either.
Of course you wouldn't expect the other characters to have uncovered a lot of evidence, or at least none that was irrefutable proof against Ivanova (or Takashima). But WE in the audience should have seen it. Except, there wasn't any.
In the end, when it's discovered that Talia was the sleeper agent/mole all along, there's been no reason to have suspected her - even for the audience's sake, let alone the other characters - any more than to have suspected Ivanova.
But if we'd seen Ivanova (or Takashima for that matter) actually involved with anything shady, in fact if there HAD been such activity that would have made the whole "Talia is really the mole!" supposed "trap door" into a big head-slapper.
All of which makes the whole "there was a sleeper agent the whole time" claim and the deliberate trap doors claims etc, seem a bit far fetched and perhaps even retcon to proclaim JMS's genius when all he really did was bring back Lyta (Patricia Tallman) when Talia (Andrea Thompson) was going to leave.