One of my favorites is to watch the dynamics that occur when someone asks to be spotted. Both parties are fun to watch. The person asking for a spot always seems to look around and ask the biggest guy in the room. The assumption is that because he's a big guy he can either remove the weight as a rescue attempt, or he actually knows how to spot (two completely different things). The other thing I've noticed is that no one ever asks for a spot on anything but a bench press. This is interesting because it ignores the fact that a significant amount of damage can be done by dropping dumbbells on your face, or hitting yourself in the head with the bar on a skull crusher. I honestly don't ever see anyone squat with a bar outside of the Smith machine even though we have a really great rack for it, so no spotting there. The other thing I rarely see is anyone asking for assistance with dumbbells or bars for loading to start position and unloading. What can be seen on a regular basis is dumbbells dropping to the floor from extended arm positions, such as behind the head on a pullover or from the extended arm position on a fly. That seems like a good method for a shoulder injury!
Back to the bench press. The big guy who has just been asked for a spot (usually acting annoyed) takes his place behind the bench. Sometimes he is watching, sometimes he's looking around. Then on the last uneven rep being gutted out, he places one hand in the center of the bar with an overhand grip to guide it back to the rack. Now, in all honesty, I've never seen an accident in the gym. I'm not sure why. Perhaps we're not lifting heavy enough to really push outside of the safe zone. Even so, if we get sloppy someone is going to get hurt. It's a good thing to know a few basics on how to spot someone for moves that are going on over the head or face.
Here are some suggestions for spotting:
Don't spot someone on a power move. You're probably not going to see explosive power moves such as cleans and snatches in your average gym. Ideally they would be performed on a platform or an adequate designated space in the case of an incomplete power move. This would allow the lifter to push the bar forward (and the body backward) to let the bar fall to the floor if necessary.
A tip when spotting on a squat or lunge from a rack is that the person should step backward at the beginning of the set and forward at the end of the set (never stepping backward at the end of a set to try to re-rack the bar). If two people are available, place a person on each end ready to grab the end of the bar. This takes a little more synchronization.
An essential component when spotting any move is communication between the lifter and the spotter. Cues for assistance with loading to start, needing help during the lift and unloading after the set should be determined before the set begins. If you're spotting an experienced lifter, you will take some verbal abuse if you randomly touch or grab the bar!
For spotting a bench press, make sure your feet are firmly planted, your knees are slightly flexed, your back is flat if bending forward and you're ready to grip the bar with an alternated grip (one overhand, one underhand), about shoulder width apart, if necessary. This gives you the best positioning to secure and handle the bar in case of lifter fatigue resulting in an incompete lift. The same technique could be used for skull crushers or overhead tricep extensions with a bar.
Exception: Spotting not needed if bar is imaginary.
If spotting someone during a dumbbell chest press, don't try to grab the weight or the lifter at the elbows. Instead, be ready to grip the lifter's arms at the wrist or forearms. This places the spotter's hands closer to the weight itself and may keep the arms from falling to the sides in case of failure. The same technique might be used for spotting overhead dumbbell presses or flyes.
If in doubt, spot each other! Frequently we feel that we don't need spotting until we get into the really heavy stuff. That may be true in many cases, but a 3 pound dumbbell can knock a tooth out if your arm fatiques. The point is safety. Ask to be spotted, and learn to spot. A big guy might be able to pull a weight off of you, but a smaller person who uses even basic technique may be able to safely spot you. Better safe than sorry.