Rav, Rabba bar bar Channa and the Cut Windpipe
An animal with a partly cut windpipe comes before Rav, who leaves it undecided; Rabba bar bar Channa then rules it kosher and even buys its meat.
In Chullin 44b, the Gemara shares a story about an animal whose windpipe had been partially cut. It was brought before the great sage Rav to determine if it was kosher or a treif. Rav sat and carefully examined the wound to see if the cut extended across the majority of the windpipe. It appears that Rav initially ruled it was treif.
Later, however, the very same animal was brought before Rabba bar bar Chana. He examined it, judged the cut to be within kosher limits, declared it permitted, and even bought thirteen coins worth of meat from it!
The Gemara immediately asks: How could Rabba bar bar Chana permit something that Rav had already declared forbidden? There is a strict halachic rule that once a Torah scholar forbids something, another scholar cannot come along and permit it. The Gemara resolves the issue by explaining that Rav had never actually issued a definitive ruling; he was simply undecided, leaving the case open for Rabba bar bar Chana to rule.