Certain issurim (forbidden things) can become batel (halachically nullified) if they were unintentionally mixed in heter (permitted things). In order for this to happen, the heter must outnumber the issur . Depending on what the issur is, the necessary measurement may be 1/60 (taste), 1/100 (terumah) 1/200 (kilayei hakerem and orla) or just a simple rov (majority).
However some things "never" become batel; they are assur even b'mashehu, that is, even in the smallest amount. The classic example of this is chametz (leaven) on Pesach, which is assur even in the minutest proportions.
However, I have been told by a Rov that even mashehu has a shiur (measurement) in which it becomes batel. The example I heard was that the Kinneret's water does not all become assur just because someone dumped chametz there on Pesach. (Yes, I'm aware that some are machmir on this, but let's leave that out, shall we?)
What is this shiur? Is it just a vague "whole lot" kind of feeling? Has any Rov quantified it, or said that it cannot be thus quantified?