Never turn your back on a felling. Keep eyes in every direction, not just on the tree you are cutting.
Tiny little vines are easy to ignore, but they often grow from tree top to tree top. The tree you are dropping is often "stitched" to other trees by vines. When the tree drops, it can tug on other trees 50 feet away. Stuff, far away from the tree you are working on can come crashing down.
It's pretty easy to get caught up marveling a huge tree while it crashes down perfectly just the way you planned. But you still need to look up and all around. An adjacent dead ash tree (and we have many of those) can be left standing and shaking long after the tree you cut is on the ground. Those standing trees that remain can drop parts for a long time after you think everything is safe. KEEP YOUR EYES OPEN!