Placing the hands too low, breaking off the tip of the Xyphoid Process, for starters, I suppose (never seen that happen, actually). Maybe placing the hands too far to the left. Or not doing CPR before shocking with an AED (you need to prime the pump!). Mostly, in the heat of the moment, people either get into speed, and don't get to the proper depth, or they focus on depth, and don't go fast enough. Just as bad is when people don't rotate out, and they tire out, giving inneffective CPR (too slow AND not deep enough) for who knows how long until someone finally notices. It's exhausting, and nobody's Superman, so have a little humility and let someone else take over!
Life begins & ends cold, naked & covered in crap.