My biggest problem over the years has been overtightening bolts -- specifically, seat-post binder bolts. I have snapped more than my fair share of them, once with annoying consequences. I adjusted the height on my saddle during a ride, snapped the bolt, and had to ride 20 miles with the saddle too low.
I haven't snapped a binder bolt in a while, but carry a spare in my seatbag just in case. When adjusting my saddle height now, I count how many turns of the wrench it takes to loosen the binder bolt. Then when retightening the bolt, I turn it the same number of times. Seem to work in absence of using a torque wrench.