If you snug it up too much, the bell can bind up between the clutch and the crank. I made that mistake once, took awhile to figure out. I would ride to work, usually worked fine...but after long stretches of full throttle, I would come to a stop and the bike would stall, acting like the clutch was still engaged. Turns out after the long run, the oil bath gets suspended within the engine and doesnt lubricate the bell bushing as well. Normally it would be fine, but with the added heat of the long run, and the extra friction the overtightened nut put on the bell, it would lightly seize the bell to the crank. After a few minutes I could ride again.
I finally figured it out from a bronze colored witness mark/ring on the spacer between the bell and the crank.