If I'm fishing a slow bait while my buddy is fishing fast and his casts keep crossing my line I'll let my line sit when he's crossed me and doesn't know it. I wait until my line starts moving and give it a good jerk. Of course he'll set the hook thinking he just hooked his new PB only to find my bait on his line.
Probably my favorite one I pulled was a few years ago on a trout fishing trip at Lake Taneycomo. It was a big group of friends, many who aren't fishermen and women, so I volunteered to go out on the pontoon and help everyone catch some fish. It was mainly little kids and their grandparents, one who had been like a grandfather to me since I was 13, my dad and his annoying, loudmouth friend. My dad's friend had been running his mouth the whole way down to the lake and the entire time we'd been there about how he was going to catch the biggest fish of the trip and he could outfish anyone, anytime, any place and it was getting on my nerves. I had everyone fishing bottom rigs with powerbait and they were catching trout about as fast as they could get a line in the water. Anyone that knows anything about inexperienced anglers using powerbait for trout knows that it results in a lot of swallowed hooks. Not a issue, we were keeping them to eat anyways so I'd just cut the line and get the hooks back later. I spent most of the time helping old Bill who was wheelchair bound and hadn't even had a chance to get my line in the water because I was so busy baiting hooks, tying hooks, unhooking fish, taking pictures, so loudmouth is in the back of the boat chipping away about how bad he's outfishing me . I'd just got done retying Bill's line when loudmouth catches a dink and swings it over to me like everyone else had been doing and insist I unhook his fish for him because "somebody around here has to catch dinner." The fish had the hook swallowed so I cut his line and tossed the fish in the livewell and got a new hook to tie on. I decide to have a little fun with him and instead of tying a good knot I tie a single overhand knot and pop a couple eggs on the hook. He casts out and I watched with much anticipation. Not long before I saw the telltale "tap, tap" of the rod tip and he set the hook into what was by far the largest trout anyone had hooked. He was hooting and hollering about how he was going to win the big fish pot until it got to the boat and he tried to swing it in and the knot slipped and the fish fell back in the water. I was laughing so hard I couldn't breath and it took a minute for me to be able to explain what I'd done. Everyone else thought it was hilarious except loudmouth who insisted it wasn't fair and his fish should still be the biggest fish of the weekend because he would have caught it otherwise, everyone laughed even harder when old Bill said "what fish?"