All it takes is a small lapse in judgment to change your life. I haven't had any accidents with a saw yet. Been running one for over 30 yrs. But, I rolled a tractor on Memorial Day weekend of 2012. I was moving round bales off a field and turned on a hill with a bale too high in the air. A rear tire landed on my lower back. Pelvis was split in half, 3 broken vertebrae, and I tore the muscles off the front of my pelvis when I pulled myself out from under the rear tire. My phone was in the house, so I had to wait for wifey to come home and find me. The muscle damage didn't heal right and will bother me for the rest of my days, and I can tell when the weather is going to change because the hardware holding me together hurts like hell. Everybody says "It was an accident." But if I would have had my brain in gear, it never would have happened.
Back before i got into tree work i was doing landscaping and construction. We won a contract to do all the landscaping and fencing for a new 1.5 million liter water supply tank that was being built to be used as a backup supply for when the mains needed to be shut off for work. Anyway, one of the last days we were there, the construction company who built the tank, was putting topsoil back down over the hillside and their most experienced truck driver who was only a month from retirement forgot what he was doing and put the tipper up to dump his load but he was facing more across the hill than facing up....... yep the tipper went up and the truck went over. The cab hit his last pile of soil and the truck twisted the chassis right behind the cab. Truck was only weeks old and was a throw away. Thankfully he was unharmed, but they did make him take an early retirement.
There are many work situations, not only tree work, that are very dangerous places to be. People who are in these environments often know the right procedures and right PPE and so on. However in this mix is the fact that we are human. We make mistakes, no matter how many times we have done something before there is always the potential to stuff it up. We forget things. Forgetting the milk and bread may be a pain but it is not life threatening (note: may be life threatening after telling your wife you forgot....
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As Brad will I'm sure testify, not wearing a helmet, forgetting to look up, and forgetting to let go of the saw and back off, is however a lot more relevant in this situation.
Don't fell trees alone. Wear the PPE.....Always. The one time you don't wear the helmet because you are complaining it is too hot or whatever, is the time that persistent bugger Murphy will come and visit. Don't take shortcuts.
Also no matter how nice a saw is ported or otherwise, if it gets pinched in the cut and the tree is falling, let go of the saw and save your ass! It is only a saw. On the plus side, if it gets damaged you can always repair it or buy another!
I have had many "incidents" over the years working with trees. Only through following good working procedures and being consistent and vigilant have i been fortunate to not have been hurt seriously. It has the potential to happen to anyone.