"Huge spider on my nose, better ignore it!"
That's actually a real dilemma.
If the spider could kill you in one bite, would you hope that it gets bored and hops off your face or would you try to smack it and risk being bit? Assuming as well that the spider is too big for one smack to guarantee a kill, and small enough so that it would be difficult to knock it clean off your face in one movement.