A pot leaves your kitchen perfectly kosher and comes back forbidden. Nobody did anything wrong on purpose — the person who borrowed it simply did not know it was a meat pot.
That is the whole subject, and the remedy is one sentence spoken at the door.
Say what it is
When lending vessels to neighbours, friends or relatives, tell them the status of the vessel — meat, dairy or pareve — so they do not stumble and do not treif it by accident.
Notice that the concern runs in both directions. They should not be put in the position of using a meat pot for dairy; and you should not get the pot back needing kashering without knowing that it does.
A borrowed vessel returned silently is the real risk here. If you lend something, it is worth asking on the way back what went into it.
When not to lend at all
Where there is genuine reason to think the borrower is not careful about kashrus, it is better not to lend the vessels.
This is stated plainly and without elaboration, and it is a judgement call about a specific person rather than a general policy. The sources do not suggest interrogating anyone — they suggest that if you already know, act on what you know.
Taking vessels out of the house
The same problem runs the other way when your vessels travel: food carried to a neighbour, to friends, to a simcha, to school, to shul, or to someone in hospital.
The instruction is to keep track of the vessels and bring them home before they are mixed in with other vessels whose status nobody knows — and before other people use them in ways that would forbid them.
Anyone who has collected their dishes from a kiddush an hour late recognises the situation. A tray left on a communal counter is a tray that has been used.
Guests, and renting out your kitchen
Guests who use the kitchen need the same information the household has. So do tenants — whether you let the house for a summer, for an occasion, or long term.
What to tell them:
- Which vessels are meat and which are dairy.
- What the markings mean.
- Where each set is kept.
- The kitchen's own arrangements — the sinks, the counters, the oven.
A short walk-through on arrival does more than any amount of correction afterwards, and it is much easier than dealing with a vessel after the fact.
What to do about a vessel that has come back from a loan is a question for your rav.
Where this comes from
- Shulchan Aruch, Yoreh De'ah 89 and 93
- ספר הכשר כלים, פרק ב — ongoing maintenance of a kosher kitchen