It happened to me just the other day. That's why I'm posting a beauty picture of the Truckee river instead of a shot of a great trout.
I spent a long hot, non productive day on the Truckee around Farad.
They say they've been getting some nice trout this year.
I went, but due to my schedule I could not get on the river until nearly 11am. It was already getting hot.
I fished deep pools, hoping for big fish. But things just shut down. I even had a guide with me, and he had no luck.
My first set up
The next evening I was on the river by 7pm.
I tossed a three fly set up and got a hit almost immediately.
By 8:15 the hatch was in full swing. I saw it all. Various "stones" and other caddis. An occasional salmon fly - and a few mayfly.
Fish were rising. One "mauled" my middle fly - and made a birds nest out of my line!
I altered my set up - and despite conventional wisdom I put a blue winged olive dry as the bottom fly. I had a yellow stone dry, and a new caddis emerger I saw in the fly shop earlier that day, up above.
My second set up
I let it drift through a seam. I felt it snag on a boulder, and went to retrieve the line to save my flies. It wasn't a boulder. I had a fish on! I set the hook, but barley could tell. My stiff 6wt rod didn't react much. I reeled in all my slack, and could still feel the small trout on the line.
I decided to quit playing him and net him. Bam! The rod bent down and nearly jerked out of my hand. This was no little trout. It was a big one, and he had been swimming in my direction. Now, he was going his own way - and I could hardly stop him. He jumped and surfaced. I not only got a good look at his size, but could tell it was a nice German brown.
I also could see my top two flies - meaning he hit the BWO dry on the bottom.
And then it was over. The line slacked, and he was gone. On the verge of netting nice one, my knot slipped. I had not tied a good knot. My bad, and it cost me.
The moral of the story - learn good knots. Tie them well, and tie them tight.