My question refers to the class of permitted actions that may inadvertently cause melachas as byproducts. (For example, with hair brushing, the intention is not to pull out the hair, but this does often occur by accident; thus the melacha of shearing is performed inadvertently.)
I had thought that such actions were permitted on Shabbos so long as a byproduct melacha was not inevitable, and that in cases where the melacha is inevitable--such as hair brushing--the action is prohibited.
However, I have encountered one source that says, rather confusingly, "The halacha is that a dvar sheina mitchaven is completely permitted." Does this mean that dvar sheina mitchaven is permitted even if a resulting melacha is inevitable, and is that correct?