Kilayim, Chapter 9, Mishnah 4. The Mishnah deals with two items that contain kilayim, yet are not forbidden: "tachrichei ha'meis u'mardaas shel chamor ein bahem mishum kilayim" - the shrouds of the dead are the burial cloths in which the body is wrapped and dressed, and the mardaas is the saddle pad placed on a donkey's back.
Shrouds of the dead:
There is no prohibition of kilayim in burial shrouds, for the reason stated in Tehillim: "bameisim chofshi" - the dead are free of the mitzvos, and therefore such a garment may be used to dress them. And it is not only that there is no problem after the fact; it is permitted to do so even at the outset: one is entitled to make burial shrouds out of kilayim.
A donkey's saddle pad:
For a donkey's mardaas, meaning the saddle pad, two conditions are required:
That it be recognizable: it must be possible to see that it contains wool and linen, for otherwise there is a concern that a person will take a piece of the pad and use it as a patch on another garment, which is in fact worn.
That it is not normally wrapped around the body: even a pad whose kilayim is recognizable presents no problem, because it is a very stiff surface. It is not wrapped around a person; one only sits on top of it.
"Lo ya'aleh alecha":
Even though it is permitted to sit on the pad, "lo yitein ha'mardaas al keseifo" - one may not take it and place it on his shoulder, even in order to carry out manure and fertilizer with it, since his entire intention in doing so is only to protect his clothes from getting dirty.
The reason for this: the verse says "lo ya'aleh alecha" - a garment of kilayim may not be brought up onto one's body. Sitting on it is permitted, since that is not bringing it up onto the body; but placing it upon himself is forbidden, and even a passing use for the purpose of removing something dirty does not permit it.
In summary: in this Mishnah we learned of two cases that do not fall under the prohibition of kilayim: burial shrouds - because "bameisim chofshi," and it is permitted to make them of kilayim even at the outset; and a donkey's saddle pad - provided that its kilayim is recognizable, and because it is a stiff surface that one sits upon and that is not wrapped around the body. Nevertheless, on the strength of "lo ya'aleh alecha," it is forbidden to place the pad on one's shoulder, even for the purpose of carrying out manure.