Sandbagging is the buyer’s right to bring a warranty claim after completion for a problem they already knew about before signing. The name comes from the idea of hiding a heavy bag of sand behind your back until it is time to swing it.
Technically, it is the default position, absent express anti-sandbagging language, under which a buyer’s pre-existing knowledge of a breach does not bar a warranty claim. A pro-sandbagging clause makes this explicit and protects the buyer’s right to claim regardless of what they knew. Silence in the agreement leaves the position to be worked out under general contract principles, which is not a comfortable place for either side to sit.
The buyer wants sandbagging preserved, expressly or by silence, so knowledge gained in diligence never costs them a claim. The seller wants it switched off, because a seller who has made full disclosure should not also carry warranty exposure for the exact things they disclosed.
This is the same fight as anti-sandbagging, viewed from the other side of the table. Whichever side drafts the first form of the agreement usually starts from their preferred position and leaves the rest to negotiation, so it is worth noticing which default the first draft assumes.
What you as a seller should push back on: a silent agreement that leaves the position to be argued about later, and any suggestion that sandbagging rights are simply market standard and not worth negotiating.