Mishna Makkos 1 (5) describes the case of witness who are zomemim.
The mishna according to Chachomim (with whom the halacha goes) says
If others [i.e. another pair of witnesses] come and are proven [a prior pair] to be zomemin, and [still] others come and prove them to be zomemin, even [if this occurs] one hundred [times], all of them are executed.
This means that Set 1 are shown to not have been present when the murder took place because Set 2 says that Set 1 were with them somewhere else. Set 1 are put to death.
Later Set 3 accuse Set 2 of not being present with Set 1 because they were present with them somewhere else. So Set 2 are are put to death.
Now we look back and see that Set 1 were put to death wrongly.
Rabbi Yehudah would only put to death Set 1 because (Bartenura) he holds that after the first group is categorised as zomemim, the testimony of a second group that comes is no longer accepted, and if [a second group does in fact] testify and is categorised as zomemim, they are not killed.
How can Chachomim justify that Set 1 were put to death wrongly?