As a few others have noted, my worst experiences have been with fellow tournament fishermen.
A few years ago, our local bass club held it's top 10 fish off on Lake Lanier just north of Atlanta, GA. It is a very large lake, so there are usually several tournaments going on each weekend. This particular weekend, there was a tournament being sponsored by one of the large marinas on the lake. There were approximately 300 boats in their tournament.
The fish had been biting back in the head of the coves, but after a heavy rain and cooler weather, I found the fish biting a crank bait around the mouths of those coves on rocky banks. By 10:00 am I was culling. As I was fishing the left side of the entrance to a beautiful cove with a couple of nice docks and a large creek, a very nice Ranger bass boat almost washed me ashore just so that he could beat me into the head of the cove. I just crossed over to the other side of the cove entrance and kept fishing. With one of my next casts, I hooked a 4 lb. 10 oz. largemouth. I raised it up, turned it around a couple of times and dropped it into the livewell and returned my smallest fish back into the lake.
The other fisherman came back out of the cove with his trolling motor. He explained that he was fishing the large tournament and hadn't had a bite all day. He asked me if I could tell him what I was catching my fish with and in what areas I was finding them. Very politely, I said "NO" and kept on fishing. :-X
I ended up winning our club tournament that day with 14lbs.. The ONLY other fish caught from our club was a 2 lb. 10 oz. largemouth.