I admit that I’m surprised at how much interest there is lomdish posts based on the little poll posted on the side, but the poll is still open and results can change. Thank you to everyone who bothered to put in their 2 cents -- much appreciated. My guess is people like chakiros and also like being me'dameh milsa l'milsa and making chilukim. Unfortunately for me thinking of sugyos in those terms is very hard to do when one has a regular job taking up most of the day.
The gemara (Chulin 96) offers a proof that eyewitness recognition, teviyus ayin, is stronger proof than simanim: if two witnesses come forward and say they recognize a murderer, their testimony is enough to create a chiyav misa and cause the murderer to receive the death penalty. However, if two witnesses do not recognize the murderer, but identify him through simanim, certain marks or features, that is not sufficient to convict.
Tosfos challenges the gemara’s assertion that the death penalty cannot be given on the basis of simanin with the following case: A woman may rely on simanim to establish that her first husband is dead. If she then remarries and is not faithful to her second husband, she is guilty of adultery and punished. Without the simanim proof that her first husband is dead and she is legally married to her second husband, there would be no way to establish the woman’s guilt (you can't commit adultery if you are not legally married). Therefore, we see that simanim can provide a basis for the death penalty.
Tosfos is forced to conclude that if the death of the first husband is established either through simanim or a single witness alone the adulteress would not be punished.
Compare this Tosfos with the following chiddush of the Rambam (Hil. Sanhedrin ch 16):
אינו צריך שני עדים למלקות, אלא בשעת מעשה; אבל האיסור עצמו, בעד אחד יוחזק. כיצד: אמר עד אחד חלב כליות הוא זה, כלאי הכרם הם פירות אלו, גרושה או זונה היא אישה זו, ואכל או בעל בעדים אחר שהותרה בו--הרי זה לוקה, אף על פי שעיקר האיסור בעד אחד.
If a single witness says that Reuvain ate cheiliev, Reuvain gets no punishment -- two witnesses are needed to establish guilt. However, if a single witness testifies that a piece of meat is cheilev, and then afterwards two witnesses come forward and testify that Reuvain ate that piece of meat, Reuvain would get malkos. Even though we only have the say-so of a single witness that the meat is cheilev, since that one witness is believed before the fact, Reuvain is chayav.
Does Tosfos argue on this Rambam? Is there a difference between convicting Reuvain of eating meat we know to be cheilev based on the prior testimony of only one witness and convicting a woman of adultery based on the prior testimony of only one witness that her first husband is dead and she is legally married to her second husband?