From time to time I think my line is breaking at the fly. I think it might be the knot intself cutting the line. It has happened with different knots. Has anyone experienced this and know of a way to prevent it?
Could you be more exact please. No one I know ties the line directly to the fly. 
I mean that my tippet appears to break at the knot that attaches it to the eye of the fly hook.
I believe that there are two reasons this happens1. Friction (heat) when the knot is tied and 2. The line gets a nick in it when you tie the knot.
Friction causes heat buldup which in turn weakens the leader. By moistening the knot before tightening this is partially eliminated. Also by tying a knot with a high leader strength retention this can be lessened. There has been several threads recently on this subject.
Also you can, and I have, nicked the leader when tying the knot. This is done by not making sure that the knot is not touching that part of the eye of the hook that is slightly open. That end can be sharp.
Hopefully some others can add some thoughts to your question.
Tim
I would have to agree that the knot is probably the problem. Make sure that the entire knot is moistened before you cinch it down. It makes a world of difference…especially on light tippets.
Brian
Do any of you think it could also be due to pulling on the tag end too much or too little before pulling on the line to finish tightening the knot. I typically attach my fly with either an improved clinch or a sliding surgeons knot?
Knots break when they slip
They slip because they’re not seated correctly
They need to be lubricated and then drawn tight.
Try tightening both the standing line and the tag at the same time.
… then again… you may just have bad tippet
Slough Foot
I think Dudley may have stated your problem.
Was this always happening to the same tippet material? Or possibly different tippets but purchased at the same time?
Your tippet may be old, either already old in the store when you bought it or you bought it over a year ago and it has seen the light of day too much. If your tippet it mono then the UV light really ages it fast. I replace my mono tippet every year.
If your tippet is fluorocarbon then the UV light does not damage it, however many of the normal knots used for mono will cut into fluorocarbon.
Messing with that tippet just isn?t worth the risk of possibly loosing the Big One that you may hook up on this year. Buy some new rolls of tippet and see if they also break. If so, then it is the knot or as suggested possibly a damaged tippet when you are tying it.
Larry 
—sagefisher—
I don’t mean to head us off on a tangent but out of curiosity what knot are you using?
I check the knot every time I take goop off the fly. Give it a short tug, sometimes it will part. It may be from banging off the rocks in swift water. If you get temporarily snagged in rocky ground and it comes free quickly the knot can get damaged. Pays to check it now and then.
You might have to change the type of knot you are using. I use an improved cinch knot. The improved cinch will not come undone the regular cinch might.
MAny times flourocarbon will cut right thru monofilament. Try to use like-materials with leader and tippet.
I get this problem myself…once I came to the conclusion it was from carrying bug spray in my pouch…another was bad line as it broke wherever you pulled…some breaks were from the friction of cinching the knot down…I always look for the milky color or distresssed color at the knot even after moistening the line…the hardest one to pinpoint was hard to believe…it was a big spool of 10lb flouro and I kept breaking off fish after fish…the problem was a super small “nick” in the plastic spool itself which would put the micro cut on the line on every revolution of the tippet spool…I have never blamed the knots themselves…I have straightened hooks on bones with 8lb tippet so I know its not the knots themselves
Slough,
I have to ask. After the knot breaks is the tip of your tippet curly or straight?
If it is curly your knot is slipping. Don’t pull on the tag to tighten a clinch knot I moisten the knot and pull the standing line of the knot slowly but steadily until the knot is tight don’t stop once you start to tighten. Even moistened if you stop the heat buildup will set the tippet (from Lefty). I use the improved clinch only on thicker material 0x-4x a regular clinch 5x-9x. I’ve never heard of a Sliding Surgeons Knot cold you post an example?
I don’t know what material you use but I remember the old Dai Riki material before they came out with the Velvet was one of the few that was recommended that you did not moisten before tightening the knot. I dated myself with that one.
If you tippet is straight you may be starting your forward cast to soon and be actually snapping your fly off. You may be “cracking the whip”. I do this after fishing graphite then picking up a bamboo rod.
As others have said it could be bad or old material. Or bad spools. I find the one thing that will cause tippet to go bad real fast is to have it in an outside your vest holder where sunlight can hit it at all times especially if your vest is in the car all season. I no longer use tippet spool holders unless they fit a vest pocket.
Tom
I’m really confused, that is EXACTLY where it should break! At the fly should be the weakest point. You rather lose the tippet, maybe the whole leader? You can’t improve on this.
actually I would have to agree with that…maybe you need to just step up the tippet… in my experiences with break offs there always been a reason and it has never been from the tippet being too small but damaged line…maybe it was just bad luck that day…good luck
I appreciate all the input on this. My break-offs have been with 6x tippet. I have discovered that I didn’t moisten the knot sufficiently and sometimes have seen line curl when breaking. After fixing that, I do think I was overtightening the tag end before tightening by pulling on the fly and tippet line. The remaining fix I’ll go for is to more carefully checking my tippet for small nicks.
Here is a link to the best knot I have found for tippet to fly.
www.flyanglersonline.com/begin/knots2/weaversknot2.php
Doug
Since were naming knots we use to tie on flies,
I use the Uni knot and its excellent.