God bless the fly fisherman who gets to see his backing and then cursed for the ones who see the arbor knot.
Ever have one slip?
The plain old arbor knot just doesn’t cut it for me. I take the knot a step further and add two clove hitches.
What do you do?
We recommend that you take TWO turns around the arbor. Mind the direction of pull too, make sure it tightens.
http://colo2.flyanglersonline.com/begin/knots/arbor.php
The only time I see my arbor knot is when I replace my backing (every 5-7 years give or take). I’ve never even come close to having a fish take all my line and all my backing too.
I’ve never gotten spooled either, but I’ve come close 
I use a regular clinch knot.
If you can see the arbor and the fish is still hot, DO NOT let the fish get the last turn out of the backing with any speed. Clamp the spool and gain line or loose the fish. It isn’t worth the fly line and all your backing. Doesn’t matter what knot you use, the backing will usually break at the arbor before your tippet parts.
Until you have had the fun of turning the reel handle and realizing that, no line is coming in… Yes, it happened to me years ago. The backing was slipping on the arbor. The whole fly line and backing just did not move as I turned the reel. THAT is why we strongly recommend you take the two turns of backing, not just one, then the arbor knot, and make sure of the direction. One way it gets tighter, the other way it wants to open and slip.
I have seen arbor knots way too often and especially when someone else was fishing my gear… I lose a line almost every year… One way or another.