The concept of דבר שאינו מתכוין means that someone drags a bench or the like and doesn't intend for it to make a forbidden ditch in the ground. Even if he does we rule that he is not liable because he didn't intend for the melacha to occur. He intended to perform the act of dragging the bench but not for the melacha to occur and is thus not liable.
There is another concept called מתעסק which means that one intends to do a permitted act and unwittingly does a forbidden act such as intending to cut a fruit that was not connected to the ground and unwittingly cutting [some say a different fruit, some say the very same fruit] one that WAS connected to the ground. Such an act ALSO doesn't incur liability, as the gemara derives both from a pasuk and from the fact that it is not a מלאכת מחשבת.
Asks Reb Elchonon [the real one, from Baranovitch, not the fake one from the Old City and Givat Ze'ev] - why do we need the new concept of מתעסק to free this dude from liability when every מתעסק is ALSO a דבר שאינו מתכוין??? Why is מתעסק not subsumed under the category of דבר שאינו מתכוין??