Kilayim, Chapter 7, Mishnah 3. This mishnah lists a series of places where one may not sow to begin with, and yet, if one did sow there, the sowing does not create a prohibition and does not render what grows there forbidden as kilei hakerem. In the words of the mishnah: "Elu asurin ve'lo mekadshin" - it is forbidden to sow in them, but the sowing does not create a prohibition, and whatever grows there is not forbidden.
These are the places listed in the mishnah:
Mosar churban hakerem (the leftover space of a ruined vineyard): At the beginning of Chapter 4 we learned that the area of a vineyard that has been ruined must be sixteen amos, with four amos set aside on each side. If there are not sixteen amos there, one may not plant in it. Suppose there are only fifteen amos: four amos are set aside on each side of the vineyard, and in the middle seven amos remain instead of eight. That middle area is the "mosar churban hakerem" - one may not plant in it, since there are not a full sixteen amos between the two sides; and if one did plant in it, it does not render the produce forbidden.
Mosar mechol hakerem (the leftover space of the vineyard's border): In the case of a vineyard adjoining a fence, twelve amos are required - four amos next to the vineyard, four amos next to the fence, and four amos remaining in the middle, in which it is permitted to sow. But if the middle area is smaller than that, this is "mosar mechol hakerem": one may not sow in it, and if one did sow, it does not render the produce forbidden, and what grows there is not prohibited.
Mosar piskei aris (the leftover space of a break in an espalier): A vineyard growing on an espalier that is broken in the middle. We learned that if there are five vines on one side and five on the other, and in the middle eight amos and a bit more - one can separate from the vineyard on both sides and sow in the middle. (How much space must be given is a separate question.) The middle area, whether large or small, is "mosar piskei aris": if it does not contain eight amos and a bit, one may not sow in it to begin with, and if one did sow, the produce is not rendered forbidden.
Mosar apiforos (the leftover space of a trellis): A lattice, or the poles projecting from the lattice on which the vine is destined to climb - one may not sow beneath them, and if one did sow, the produce is not forbidden, provided that no actual vine is growing above them.
In contrast to these, the places that do render produce forbidden:
Beneath the vine itself.
The working area of the vine - within the six tefachim adjacent to the vine.
The four amos of the vineyard - within the four amos adjacent to a complete vineyard, whose allotted space is four amos.
In these three places the sowing does create the prohibition of kilei hakerem, meaning: the produce that grows there becomes forbidden.
In summary: This mishnah lists four places where it is forbidden to sow to begin with but where sowing does not create a prohibition - mosar churban hakerem, mosar mechol hakerem, mosar piskei aris and mosar apiforos; and in contrast, three places where sowing does render the produce forbidden - beneath the vine, the working area of the vine, and the four amos of the vineyard.