Moed Katan, Chapter Two, Mishnah 3. This mishnah deals with the permission the Sages granted to perform certain kinds of work on Chol HaMoed in order to prevent a loss, and with the limitation they placed on that permission: a person may not deliberately schedule his work for the days of the festival.
"Machnis adam peirosav mipnei haganavim" - a person may bring in his produce because of thieves:
Someone whose produce is lying outside, in a garden or an orchard, after it has been harvested, may bring it in on Chol HaMoed out of concern that it will be stolen. The Gemara adds that he should do so as discreetly as possible, since the permission was granted solely to prevent a substantial loss.
"V'sholeh pishtano min hamishrah bishvil shelo sovad" - and he may draw his flax out of the soaking vat so that it not be ruined:
Flax that is lying in water as part of the process of preparing it, which will be ruined if it is not taken out, may be removed from the soaking vat on Chol HaMoed so that it not be lost.
"Uvilvad shelo yechaven es melachto bamoed" - provided that he does not intend his work for the festival:
A condition applies to all of these permissions: that the person did not arrange his affairs in such a way that he would be able to do these tasks specifically on Chol HaMoed, when his hands are free.
"V'chulan im kivnu melachtan bamoed - yovdu" - and in all these cases, if he intended his work for the festival, they shall be lost:
This rule applies to every case in which the Sages permitted work because it involves something that would otherwise be lost, that is, something whose neglect would cause a loss. One who acted against the will of the Sages and intended his work for the festival, arranging that the work be done specifically on Chol HaMoed, is subject to the ruling of "yovdu", and he forfeits those very items.
The Rishonim disagree about the meaning of "yovdu":
Some say that the item must be left in the state in which it is, and he is not permitted to continue and complete the work.
The Rambam explains that beis din actually destroys the items or declares them ownerless, so that they belong to no one, as a penalty for what he did.
Rashi understands that "yovdu" means that they become forbidden to derive benefit from, and he may not benefit from those very objects on account of which he violated the sanctity of Chol HaMoed by intending his work for the festival.
In summary: in this mishnah we learned that it is permitted to bring in produce because of thieves and to draw flax out of the soaking vat, in order to prevent a loss and provided that it is done discreetly. But one who intends his work for the festival is penalized by the rule of "yovdu", and the Rishonim disagree as to its meaning: leaving the work unfinished where it stands, having beis din declare the objects ownerless, or a prohibition against deriving benefit from them.