Chullin 43: Examining an Esophageal Injury After Slaughter

If an animal's esophagus is injured but a membrane grew over the injury, it is considered to be a tereifa because the injury was severe.  The injury can be checked only from looking at the inside of the neck, and thus an external examination is not valid in this type of case.  

We are told the story of a bird which may have been damaged in this way.  The esophagus was examined from the outside by Rabba.  Abaye asked him if he did not say that the gullet must be examined from the inside.  Rabbi the turned it inside out and examined it.  Finding two drops of blood, he declared it a terrify.  The Gemara says that Rabba did not make an error; he was testing Abaye.