I heard that placing hot pots onto counters or refrigerator shelves could treif the pot (although not its contents) if the counter or shelf is dirty with an opposite type of foodstuff. Is there a halachic source for this?
I heard that hot pots should be placed on a towel, board or trivet. Is there a halachic source for this?
I heard that hot corning or plastic coming out of the microwave could create the same problem. Is there a halachic source for this?
Here's a quote from the Rema YD 92:7:
ואם נשפך חלב או שאר איסור רותח על גבי קרקע והעמידו עליו קדירה חמה אם מה שנשפך אינו אצל האש לא הוי רק כלי שני ולכן הקדירה אסורה דבולע קצת והתבשיל מותר דתתאה גבר
My Loose translation:
And if [hot] forbidden fat (or any other hot forbidden food) spilled on the ground, and one put a hot pot on it, if what spilled is not near the fire, it is only a kli sheini, and therefore the pot is forbidden, because it absorbed a bit, and the food is permitted, because the bottom is stronger. (In other words we look at it like a kli sheini like the bottom-what spilled, not a kli rishon like the top-the pot)
Although the Rema was talking about a hot spill and the asker was asking about cold things on the shelf, the law is the same: the pot absorbs a bit and is forbidden and the food is permitted.
So why does the Rema talk about a hot spill? He does because the Issur Vheter (see Darchai Moshe 92:7) argues and also forbids the food in that case, and the Rema is coming to argue on that.
(It's not totally clear that the Issur Vheter doesn't forbid also by a cold spill....see Minchas Yakov that he only forbids a hot spill (because we dont say the bottom cools off the top if the bottom is hot too, even if it's only a kli sheini) and Yad Yehuda that he forbids a cold spill too (because we only apply the rule that the bottom cools the top if the top doen't have something else on top of it keeping it hot-here the pot has the hot food in it). Either way, the Rema doesn't paskin like him...uvenai yisrael yatzu beyad Rama...)
As always, ask your LOR for practical guidance.