Injured my shoulder today. Since typing is kind of painful I'm just going to copy-and-paste the explanation from GameFAQs:
Today I was doing Arnold presses (http://ironechelon.webs.com/ArnoldPress.htm) with 85 lb. dumbbells. Usually this doesn't pose much trouble but I'd just done 8 sets of dips (working up to 135 lbs.) and fatigue was a factor - consequently, it felt harder than I expected it to. I managed 5 reps, but on the 6th, as I struggled overhead, I suddenly felt the left dumbbell fall backward...with my arm in tow. I heard a CRUNCH in my shoulder as the humerus popped out of its socket. The weight fell on the ground and my arm hung loose as it somehow popped itself back in. What a wuss. I was so ashamed of it, I shunned the other limp arm as I continued lifting with my good one. Then I decided to end things there.
Not to state the blatantly obvious, but more as a reminder: lift heavy or lift more reps (though still moderately heavy, just enough for the rep range), but if you ever try to combine the two and really attempt to power every rep, don't push it when you start having trouble. This is how I've injured myself in every single instance. It always happens on the last rep and, except for this time, on the last set.