The mishnah on Makkos 7a states:
סנהדרין ההורגת אחד בשבוע נקראת חובלנית רבי אליעזר בן עזריה אומר אחד לשבעים שנה רבי טרפון ורבי עקיבא אומרים אילו היינו בסנהדרין לא נהרג אדם מעולם רשב"ג אומר אף הן מרבין שופכי דמים בישראל:
A Sanhedrin that executes once in seven years is characterized as a destructive tribunal. Rabbi Elazar ben Azarya says once in seventy years. Rabbi Tarfon and Rabbi Akiva say: If we had been of the Sanhedrin, no person would have ever been executed. Rabban Shimon ben Gamliel says: if that's the case, they too would increase the number of murderers amongst the Jewish people.
The mishnah on Sanhedrin 81b teaches us about the concept of a "kippah"- a small chamber where a murderer who evaded a death sentence due to technical reasons (no witnesses or hasra'ah etc) would be put to death.
ההורג נפש שלא בעדים מכניסין אותו לכיפה ומאכילין אותו (ישעיהו ל, כ) לחם צר ומים לחץ:
one who kills a person not in the presence of witnesses [and it is impossible to judge him in court], the court places him into a vaulted (ie tiny) chamber and feeds him sparing bread and scant water (see Isaiah 30:20).
The Rambam (Hilchos Rotzeach 4:8) explains what would happen:
All of these murderers they would bring to a kippah and feed him a small amount bread and scant water until his intestines contract and then the court feeds him barley that expands in his innards until his stomach explodes.
In light of the concept of kippah, how in the world does the mishnah in Makkos make any sense?
There'll never be a problem of murderers as they would STILL be put to death via this method! (statistically speaking, it's probably likely a murder will occur at least every 7 years)
NOTE: perhaps there's room to argue that the kippah method is different than a literal court imposed death penalty as it's via gramma (indirect causation)- nonetheless whether directly or via gramma the Sanhedrin STILL end up putting a person to death.