In my previous 'birth years and Zodiac animal signs for Weiss Kreuz three assassin teams' post, murasakisilver pointed out
"If the characters' ages are current as of the pilot episode, Omi was born on February 29 in a year that didn't have one," 1981.
Dar went on to ponder
"if there's anything in the series that links the February 29th birthday to Mamoru Takatori, or only to "Omi Tsukiyono"?"
And, barring an answer that no, it's linked to both names, I came up with this possible "No Prize" explanation. Warning: it's heavy on melodrama and soap opera cliches.
If you're right -- the birthday belongs to "Omi Tsukiyono" not "Mamoru Takatori" -- consider this:
* While Mamoru was the offspring of Kikuno, the woman loved by T. Shuuichi, he is also the child of his hated brother, T. Reiji. -- Or so he thinks.
* Mamoru's kidnapping & disappearance is the final step that pushes Kikuno over the edge in her loveless marriage, leading her to commits suicide.
So Shuuichi could not only hate Mamoru because of who's son he thinks he is, he could also blame him for his mother's death. (Her death could be the bit that drives the thought from Shuuichi's mind that Mamoru is also Kikuno's son.) Of course, this could be subconscious, but it doesn't have to be.
* After the kidnapping and rescue, Mamoru is completely in T. Shuuichi's hands: Shuuichi can completely rewrite not only Mamoru's personality (making him an assassin for use against that hated brother) but his existence.
* Later, for the rest of Weiss, he removes all records of their official existence.
So why not go one more step with Omi and give him a new existence that on closer examination, ultimately doesn't even exist, a way of denying that Kikuno was Reiji's wife and not his.
There also could be a desire to rewrite the past: if Mamoru didn't exist, then Kikuno could have left Reiji and been with him, Shuuichi. At the very least, he thinks, Mamoru is evidence that Kikuno and Reiji had carnal relations, that Reiji "sullied" her.
And underlying all this: the Takatoris are a warped bunch - not only Reiji, but also his sons and his father (Omi's grandfather), Saijou. Why should Shuuichi be any different? He got the same upbringing Reiji did: he doesn't see people, he sees pawns and worse, thinks of only how they can best serve him. The difference between the two brothers is the vocabulary used to cloak their actions.
I don't think Shuuichi was ever the "benevolent uncle" to Omi he is often portrayed as: I think he was consumed with seeking revenge on his brother and woe to anyone who got in the way. Omi was a living reminder that Kikuno wasn't his, and IMHO he hated Omi, seeing only a tool he could use against his brother. And it was designed to be a very cruel choice of tool, too. When I say "too bad it backfired on him", don't believe me at all, I'm being sarcastic. Shuuichi deserved what he got.
(On a related note: does anyone know if Kikuno is also mother to Hirofumi and Masafumi? Due to the age gap and the romance between Kikuno and Shuuichi, it doesn't "feel" like she is. It very much smacks of a second marriage. I can even see
* Shuuichi falling in love and romancing Kikuno;
* Saijou and Reiji deciding Reiji needed another wife to replace his deceased one (mother to Hirofumi and Masafumi, dead in childbirth?) since he's running for public office;
* them deciding Kikuno made the perfect candidate due to family and background (again, guessing at her family/background);
* Kikuno agreeing to the marriage for her family's sake/honor (it's both Saijou's and Reiji's style to use blackmail/manipulation);
* knowing it would be loveless, her choosing to have one last (first?) night with Shuuichi so she would have at least have memories of love to sustain her.
* I can even see her using a loophole of not giving her virginity to anyone other than a Takatori, too. -- Totally guessing at the "virginity" part, based on patterns for these kind of stories.)
Make sense? Comments? Alternate theories? (I already gave my answer on how I think this goof occurred in real life, aside from the usual sloppiness associated with Weiss.)