Choppy: You are probably thinking of the Shittas Harashba brought down in the Beis Yosef (if I recall correctly) that for a Tashlumin the second Shmoneh Esrei must be while you are still Osek in that T’fillah, so it could be that as soon as you move on to Tachanun is too late. The Rashba isn’t brought down by most other Poskim, but we are probably still Choshesh for it because it’s a Safek Brachos issue. But that’s not true by if you actually weren’t Yotzei.
(You might also have been thinking of the fact that even though if you’re unsure if you said Ya’aleh V’yavo (or Mashiv Haruach within 30 days, etc.) you have to assume that you didn’t and therefore repeat, that is only true if the Safek comes to you during Shmoneh Esrei or immediately after. If the Safek comes to you a while after (I don’t know what the precise Shiur is, maybe Toch K’dei Dibur but I have no Makor for that) then you can assume you got it right. But this doesn’t help if you are absolutely sure that you got it wrong.)
By “no way out of it” I meant that in a lot of cases of repeating Shmoneh Esrei there are minority Shittos that would say you don’t repeat and therefore you might actually not repeat because of Safek Brachos. Rabbiofberlin’s post is only one such example.
R.T.: While you are right M’ikar Hadin, I believe that many nowadays are Makpid to wear T’fillin as little as possible. We have narrowed it down to just Shacharis and maybe a little learning after. You fulfill the Mitzvah by that. Why risk ruining it by wearing it more than we are capable of doing properly?