The gemara in Sotah 2b discussed two different versions of a statement by Rabbi Eliezer: one from the mishna on 2a and one from a beraisa. In the mishna, Rabbi Eliezer states that in order for a woman to be obliged to drink the sotah water her husband must have warned her in front of two witnesses not to be secluded with a particular other man and then a single witness (even the husband himself) is believed to say that she was secluded with that man. In the beraisa, the requirements are flipped. The husband is believed on his own to state that he warned the wife, but two witnesses of seclusion are required.
The chachamim argue on Rabbi Eliezer in the beraisa that "אין לדבר סוף". In other words, if the husband never warns the wife not to seclude herself, she will do so. Then, if he gets mad at her for doing something terrible like burning the cholent, he can find tw o witnesses to her seclusion and claim to have warned her in the past, getting her in trouble. The gemara seems to accept that this and a corresponding argument are legitimate arguments against Rabbi Eliezer in both the mishna and beraisa, but it tries to determine in which case the argument is stronger. In the end the gemara concludes that the argument is stronger against Rabbi Eliezer in the beraisa because in the mishna's case there is a reason to be suspicious already:
אדרבה למשנתינו איכא עיקר התם ליכא עיקר
The Gemara asks: On the contrary, according to the opinion of Rabbi Eliezer cited in our mishna, there is a legitimate basis of suspicion with regard to the woman, as there are witnesses who saw the husband issue a warning to her, and therefore, it is understandable that the testimony of the husband may be relied upon when he testifies that she secluded herself with another man. By contrast, there, according to the opinion of Rabbi Yosei, son of Rabbi Yehuda, there is no legitimate basis to prohibit her to him, since there are no witnesses that she had been warned by her husband at all. Therefore, it may be that the Rabbis took issue only with the version of Rabbi Eliezer’s opinion presented by Rabbi Yosei, son of Rabbi Yehuda.
Translation/interpretation from Sefaria
Rashi comments there:
איכא עיקר - שכבר התחיל בעדים שלא ע"י כעס:
There is a basis - because the situation already began in front of witnesses before he was angry.
So my question is the following: Suppose I'm a bit more devious than Rashi gives me credit for. Even according to the mishna I could get my wife in trouble. When I get mad at her for burning the cholent, I can just summon two buddies and warn her in front of them. Then the next day I can claim that I saw her seclude herself, even though it's not true. This doesn't seem any more difficult than the case of the beraisa, so why doesn't the gemara (according to Rashi's understanding) consider that possibility? Another way of asking the question is, why does Rashi assume that the warning occurred before the husband was angry?