Make sure your tail holds and guy stumps are solid and capable of with standing the strain. Not many that I have seen will walk an extra 100 feet to a differnet hold over the top and let some earth take some of the strain and you still get the same lift and sometimes even just a bit more.
Make sure your twisters are locked in tight as a bull back side at fly time and the stick is strong enough not to snap. Nothing like your tension going slack at just the right time.
Drum line doubled up and then tied in an over hand knot, choked to a stump and then pulled tight.
Weak hydraulics or some problem with the graple. Close'em up then wrap a molly around them after they are closed and the load is lifted just enough to get the molly on so the jaws don't open on the return run.
Working with the office jockey boss wearing the bug when you have a new engineer than you aren't used to at the controls.
A new Hook from the outside tring to show good. No matter what the cost.
Bear traps.
If your going to be the one climbing, have your own gear. Or be the guy that takes care of it so you know what you have when the time comes. Nothing like being 60'+ up and having your spur bust off at the leg iron. Don't come down, set your riggin and then come down so you don't have to do it twice. Forget the rope and go with chain and a slip catch. Easier to adjust the length and chain doesn't seem to want to be as sticky as rope most of the time, chain will also bust some of the smaller limbs that rope will just hang up on.
Mostly, lack of basic maintenance and lack of good old fashioned common sence.
Stay the right size for your britches, keep your head on tight and your witts about you. After that there is a very small percentage that will happen that cannot be forseen. For that small percentage of nasties the above mentioned will be your best allies on keeping you alive and in one piece.
There are some places you have no other choice than to be, like Bushler under his cat, or Hammer cutting under that JD. Some situations are there and it just plain ole sucks to be there. The old saying, spit happens, get to work.
Owl