Rabbi Eliezer HirschKosher Without the K™

(1) Laws of Buying and Selling a טלית. Containing 2 Se'ifim:
One who buys a טלית with ציצית on it from a Jew or from a non-Jewish merchant [and he says that he bought it from a Jew, and he's believed] (Nimukei Yosef - Laws of ציצית) it is valid, because since he's a merchant, there's a presumption [that he's telling the truth and] that he bought it from a Jew because he wouldn't harm himself (ie. ruin his career by lying to customers). But, if one bought it from a non-Jew that's not a merchant, it is invalid.

(10) One who leaves a non-Jew alone in his house, and inside there are things that could be switched, and they would be then forbidden by the Torah...But if he will not benefit from the switch, it is permitted in any case, for we are not concerned that perhaps he will switch them and cause the owner to stumble, since he gains no benefit from this matter.

Utensils used by non-Jews are assumed to be unused that day. Therefore if you cooked using their utensils before they were kashered the food cooked in them is permissible.

(4) (4) Lemon juice made by gentiles and cut pieces of salted fish that are brought by gentiles in barrels are permitted. Rema: Because they bring large quantities at once and even if some of them are prohibited, for they were cut first with a non-kosher knife, they are nullified amidst the others that are subsequently cut, for the ones cut later do not become prohibited for the taste in the knife was nullified in the first items cut and so the entire group is permitted. And so too all cases of this sort.

It is permissible to plant vegetables under an asheira tree, (which was used for idol worship) during the summer when they require shade or during the winter months. This is because the shade from the asheira tree, which is prohibited to benefit from, and the land, which is not prohibited, both contribute to the vegetables' growth. The principle that permits this action is that when something prohibited combines with something permissible to produce an outcome, the result is permissible in any halachic situation...

(1) Do not roast kosher meat with non-kosher meat or meat of an unclean animal in the same oven, even if they do not touch one another. But if you did roast them, this is permitted - and even if the piece of forbidden meat was very fatty and the piece of permitted meat was lean. But if the oven is large enough to hold 12 esronim (about 26-52 liters. My note: most standard wall ovens have a capacity ranging from about 60 to 90 liters, while commercial ovens are much larger), and its mouth is open, then it is permitted to roast them in it as long as they do not touch one another. And if one of them is covered with a bowl or with dough or the like, it is permitted to roast them, even in a small oven with a closed mouth. Comment: The same is true for meat with milk - the ruling is also thus. And it is customary to be strict before the fact, even with a large oven, and after the fact to be lenient even with a small oven...