Commentary for Avodah Zarah 78:19

The fear with milk, meat and cheese seems to be that kosher ones were switched with non-kosher ones. The fear with blue wool is that the non-Jew switched the wool with wool dyed with dye not kosher for tzitzit.
The hiltit may have been cut with an unkosher knife. The murias (fish hash) may have been switched with murias that has non-kosher fish in it. The wine may have been touched by the non-Jew, which would render it undrinkable. The issue with the bread is that it may have been switched with bread made by non-Jews, which is forbidden.
Rav requires two seals for some of these things, but only one seal for the others

The mishnah allows a Jew to buy a tarit fish which has not been minced from a non-Jew and eat it. The Talmud explains that the fish’s head and backbone must be recognizable. This is how we can be sure that a non-kosher fish did not get in there. The mishnah allowed “brine containing fish” as long as that fish was a specific fish, called the kalbit fish. This fish is a sign that the brine is kosher. In open barrels two of these fishes are necessary, but in closed barrels, one is sufficient.