Rav Omri Kraus of Kerem b'Yavneh raised an interesting sefira question. Two halachos you need as background:
A) If a person is not sure whether he remembered to leave out mashiv ha'ruach after the first day of pesach (same rule would hold true for when we have to add mashiv ha'ruach after shmini atzeres), until 30 days have passed we assume the person davened as he had in the past and said it -- chazakah.
B) There is a machlokes between BH"G and all other Rishonim whether a person who misses counting one night of the omer may continue counting with a bracha (most Rishonim) or not (BH"G). B'pashtus, the issue is whether sefira is one mitzvah to count 49 days, or whether counting each day is an independent mitzvah. The halacha is to continue counting, as according to most Rishonim he can still fulfill the mitzvah, but not say a bracha out of consideration for the BHG's view.
What if a person is not sure whether he missed a night or not? The Terumas HaDeshen paskens that in this case he can count with a bracha, as there is a sfeik sfeika: safeik whether he really missed a night or not, and even if he did miss, safeik whether the halacha is like the majority of Rishonim or like the BH"G.
Question: don't these two halachos conflict? In the first case we say until 30 days have elapsed, there is a chazakah that you missed saying the right thing. So when it comes to sefira, if you have a safeik whether you counted, why not say that until 30 days have gone by you have a chazakah that you did not count? Why do we say sfeik sfeika instead of ruling based on chazakah?
There are a few ways to approach this. I won't give away the answers, but will give you a link to the shiur:
On the same topic of sfeik sfeika, the one halacha everybody seems to know with respect to sefira is that if someone asks you what day to count, once it is bein ha'shemashos you should say "yesterday was X" because if you say what night it is, you have been yotzei the mitzvah and won't then be able to count with a bracha.
At least during bein ha'shemashos, even if you told your friend what night it is, why not say sfeik sfeika -- safeik whether it is still day and so even if you said what night it is, the statement is meaningless, and even if it night, safeik whether mitzvos tzrichos kavanah (see the GR"A and Biur Halacha on that din!) and you didn't have kavanah to be yotzei? Why can't you count with a bracha in this case?