Avodah Zarah 72 - Pouring the Wine
If a Jew took a funnel and measured wine into an idol worshipper's flask, and then took the funnel and measured wine into a Jew's flask, then the rule is as follows: if at the bottom of the funnel there is a small wine retainer, that is, if the bottom of the funnel has a protruding lip which retains a drop of the wine poured through in its previous use, then the wine that was poured through the funnel in the Jew's flask is prohibited.
Those who consider a stream connecting two vessels as an actual connection, transmitting the prohibition, explain that the bottom of the idolater's flask contained minute amounts of libation wine. Through the stream connection, the wine in the funnel also become prohibited, and through it - the next wine that was poured. Those who say that the stream is not a significant connection, explain that the funnel was touching the wine on the bottom.
If an idolater pours wine from a bottle into a cup, the wine in the cup is prohibited, but the wine remaining in the bottle is permitted.
Art: Joaquin Sorolla y Bastida - Elderly Castellano Pouring Wine