i got my DOHC s13 towed to my house the other day when it wouldn't start. i took off the valve cover to check the cam timing and the exhaust cam sprocket somehow twisted. i didn't notice it from looked at the front, but from the top, the cam sprockets were not parallel. its hard to explain.
it looked like this from the top-
intake sprocket on the left, exhaust on the right
= \\
except the exhaust sproket was only tilted like 1/4 that much. this obviously is the reason my car would not start.

so i took off the bolt, straightened the sprocket, made sure the pin was in the slot, and tightened the bolt back to 130 foot pounds. at the suggestion of a nissan tech, i took off the first bearing cam and the freaking camshaft was busted completely in half. what the hell? how does a camshaft break? anyone know what i can do to prevent this from happening again?

Holy shit, never heard that one before. Not too sure, but I do know that excessive heat and improper lubrication will turn metal into peanut brittle. Maybe the bearing failed , which caused the cam to drag. This could have fuxed your sprocket up too, and caused a bunch of heat and wear, enough to make your cam brittle enough to snap like that.