I had this happen at the Allegheny Reservoir once while fishing for crappies. There was a steep shoreline along the north side of the Kinzua arm of the lake. Waves would erode the shoreline and big patches of trees would slide down the slope and into the water. One very foggy morning my dad and I motored across from the Kiasutha campground to that side of the lake and were fishing a patch of trees that had slid into the lake. It was very still and the water was clear and smooth as glass.
It was a slow morning, and after a couple of hours of fishing small fathead minnows just off the bottom above the tree roots, I finally hooked into a crappie, and I said to Dad "Here we go, I got one." I was savoring the fight, and as it was maybe 6 or 8 feet down, I saw its side flash and could see it was a decent crappie, maybe 10 inches or so. I was pretty happy with that and when it got even closer, I could see this big musky circling and gliding in from behind my fish. He just opened his mouth, flared his gills, and sucked my crappie into his mouth in one gulp. I said something like "Holy crap, a musky just ate my crappie!!!
Here I am, fishing with this little cheap Zebco closed-face fishing reel, a lightweight rod, 10 pound test line, a #6 hook and a small split shot. I fought him for what seemed like a couple of minutes, but I sure it was much shorter than that, and he finally broke me off. Dad and I both sat there and looked at each other in stunned silence trying to process what had just happened. I don't remember our exact next words, but I think it was something like "WOW, wait till we get back and tell Mom about this!!!!" That fateful day was the day my Dad and I caught the musky fishing bug. We spent a lot of time trolling that lake and caught a few muskies. My biggest was 43 3/4" and 20 1/2 pounds. Fun times.