Never sit on a pillow, because creatures will grow in your butt
The literal translation of this proverb is, On the pillow, if you sit, creature pimples will be. In India, more often than not, one is sits on the ground. Typically there are mats on the ground, and a top the mats sit pillows. Although, the purpose of the pillows are not to cushion ones rear, rather, they are to cushion ones back: the pillow is to be placed against the wall so a person my comfortably rest against it while sitting on the ground.. This little saying, is quite clearly recited between anyone, but particularly to children to remind them that the pillow is not for sitting on, but against. The informant admitted that even now, he will never sit on a pillow under any circumstances. The image is fantastically unpleasant, particularly because a pillow is, quite literally, a very plausible nesting area for bugs and creatures and anything that may have been crawling on the ground beneath it. This, in conjunction with that perfect plausibility of a creature of this sort crawling up ones behind, is a thought unpleasant enough to discourage anyone from taking this risk.
I think it is also to note that the translation is creature rather than bugs or insects. The ambiguity leaves the recipient of this warning to the no limits but his own imagination. There is a certain, mythical element here, the creatures could be anything horrible, appealing to anyones idiosyncratic phobia.